"Minus" features are applied after "plus" features, so ensure that a
later "plus" feature causes an earlier "minus" feature to be removed.

This has no effect on the existing "-feature,feature=on" backward
compatibility code (which warns and turns the feature off).

Signed-off-by: David Edmondson <david.edmondson@oracle.com>
---
 target/i386/cpu.c                   | 33 +++++++++++++++++++++++------
 tests/qtest/test-x86-cpuid-compat.c |  8 +++----
 2 files changed, 30 insertions(+), 11 deletions(-)

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 35459a38bb..13f58ef183 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -4750,13 +4750,32 @@ static void x86_cpu_parse_featurestr(const char *typename, char *features,
         GlobalProperty *prop;
 
         /* Compatibility syntax: */
-        if (featurestr[0] == '+') {
-            plus_features = g_list_append(plus_features,
-                                          g_strdup(featurestr + 1));
-            continue;
-        } else if (featurestr[0] == '-') {
-            minus_features = g_list_append(minus_features,
-                                           g_strdup(featurestr + 1));
+        if (featurestr[0] == '+' || featurestr[0] == '-') {
+            const char *feat = featurestr + 1;
+            GList **remove, **add;
+            GList *val;
+
+            if (featurestr[0] == '+') {
+                remove = &minus_features;
+                add = &plus_features;
+            } else {
+                remove = &plus_features;
+                add = &minus_features;
+            }
+
+            val = g_list_find_custom(*remove, feat, compare_string);
+            if (val) {
+                char *data = val->data;
+
+                *remove = g_list_remove(*remove, data);
+                g_free(data);
+            }
+
+            val = g_list_find_custom(*add, feat, compare_string);
+            if (!val) {
+                *add = g_list_append(*add, g_strdup(feat));
+            }
+
             continue;
         }
 
diff --git a/tests/qtest/test-x86-cpuid-compat.c b/tests/qtest/test-x86-cpuid-compat.c
index 7ca1883a29..6824d2b13e 100644
--- a/tests/qtest/test-x86-cpuid-compat.c
+++ b/tests/qtest/test-x86-cpuid-compat.c
@@ -171,18 +171,18 @@ static void test_plus_minus_subprocess(void)
     char *path;
 
     /* Rules:
-     * 1)"-foo" overrides "+foo"
+     * 1) The later of "+foo" or "-foo" wins
      * 2) "[+-]foo" overrides "foo=..."
      * 3) Old feature names with underscores (e.g. "sse4_2")
      *    should keep working
      *
-     * Note: rules 1 and 2 are planned to be removed soon, and
-     * should generate a warning.
+     * Note: rule 2 is planned to be removed soon, and should generate
+     * a warning.
      */
     qtest_start("-cpu pentium,-fpu,+fpu,-mce,mce=on,+cx8,cx8=off,+sse4_1,sse4_2=on");
     path = get_cpu0_qom_path();
 
-    g_assert_false(qom_get_bool(path, "fpu"));
+    g_assert_true(qom_get_bool(path, "fpu"));
     g_assert_false(qom_get_bool(path, "mce"));
     g_assert_true(qom_get_bool(path, "cx8"));
 
-- 
2.29.2

Hi,

Thanks for the patch.  Getting rid of special -feature/+feature
behavior was in our TODO list for a long time.

On Tue, Jan 19, 2021 at 02:22:06PM +0000, David Edmondson wrote:
> "Minus" features are applied after "plus" features, so ensure that a
> later "plus" feature causes an earlier "minus" feature to be removed.
> 
> This has no effect on the existing "-feature,feature=on" backward
> compatibility code (which warns and turns the feature off).

If we are changing behavior, why not change behavior of
"-feature,feature=on" at the same time?  This would allow us to
get rid of plus_features/minus_features completely and just make
+feature/-feature synonyms to feature=on/feature=off.

> 
> Signed-off-by: David Edmondson <david.edmondson@oracle.com>
> ---
>  target/i386/cpu.c                   | 33 +++++++++++++++++++++++------
>  tests/qtest/test-x86-cpuid-compat.c |  8 +++----
>  2 files changed, 30 insertions(+), 11 deletions(-)
> 
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index 35459a38bb..13f58ef183 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -4750,13 +4750,32 @@ static void x86_cpu_parse_featurestr(const char *typename, char *features,
>          GlobalProperty *prop;
>  
>          /* Compatibility syntax: */
> -        if (featurestr[0] == '+') {
> -            plus_features = g_list_append(plus_features,
> -                                          g_strdup(featurestr + 1));
> -            continue;
> -        } else if (featurestr[0] == '-') {
> -            minus_features = g_list_append(minus_features,
> -                                           g_strdup(featurestr + 1));
> +        if (featurestr[0] == '+' || featurestr[0] == '-') {
> +            const char *feat = featurestr + 1;
> +            GList **remove, **add;
> +            GList *val;
> +
> +            if (featurestr[0] == '+') {
> +                remove = &minus_features;
> +                add = &plus_features;
> +            } else {
> +                remove = &plus_features;
> +                add = &minus_features;
> +            }
> +
> +            val = g_list_find_custom(*remove, feat, compare_string);
> +            if (val) {
> +                char *data = val->data;
> +
> +                *remove = g_list_remove(*remove, data);
> +                g_free(data);
> +            }
> +
> +            val = g_list_find_custom(*add, feat, compare_string);
> +            if (!val) {
> +                *add = g_list_append(*add, g_strdup(feat));
> +            }

I believe we'll be able to get rid of plus_features/minus_features
completely if we remove compatibility of "-feature,feature=on".
But if we don't, wouldn't it be simpler to replace
plus_features/minus_features with a single list, appending items
in the order they appear?

> +
>              continue;
>          }
>  
> diff --git a/tests/qtest/test-x86-cpuid-compat.c b/tests/qtest/test-x86-cpuid-compat.c
> index 7ca1883a29..6824d2b13e 100644
> --- a/tests/qtest/test-x86-cpuid-compat.c
> +++ b/tests/qtest/test-x86-cpuid-compat.c
> @@ -171,18 +171,18 @@ static void test_plus_minus_subprocess(void)
>      char *path;
>  
>      /* Rules:
> -     * 1)"-foo" overrides "+foo"
> +     * 1) The later of "+foo" or "-foo" wins
>       * 2) "[+-]foo" overrides "foo=..."
>       * 3) Old feature names with underscores (e.g. "sse4_2")
>       *    should keep working
>       *
> -     * Note: rules 1 and 2 are planned to be removed soon, and
> -     * should generate a warning.
> +     * Note: rule 2 is planned to be removed soon, and should generate
> +     * a warning.
>       */
>      qtest_start("-cpu pentium,-fpu,+fpu,-mce,mce=on,+cx8,cx8=off,+sse4_1,sse4_2=on");
>      path = get_cpu0_qom_path();
>  
> -    g_assert_false(qom_get_bool(path, "fpu"));
> +    g_assert_true(qom_get_bool(path, "fpu"));
>      g_assert_false(qom_get_bool(path, "mce"));
>      g_assert_true(qom_get_bool(path, "cx8"));
>  
> -- 
> 2.29.2
> 

-- 
Eduardo

On Tuesday, 2021-01-19 at 10:20:56 -05, Eduardo Habkost wrote:

> Hi,
>
> Thanks for the patch.  Getting rid of special -feature/+feature
> behavior was in our TODO list for a long time.
>
> On Tue, Jan 19, 2021 at 02:22:06PM +0000, David Edmondson wrote:
>> "Minus" features are applied after "plus" features, so ensure that a
>> later "plus" feature causes an earlier "minus" feature to be removed.
>> 
>> This has no effect on the existing "-feature,feature=on" backward
>> compatibility code (which warns and turns the feature off).
>
> If we are changing behavior, why not change behavior of
> "-feature,feature=on" at the same time?  This would allow us to
> get rid of plus_features/minus_features completely and just make
> +feature/-feature synonyms to feature=on/feature=off.

Okay, I'll do that.

Given that there have been warnings associated with
"-feature,feature=on" for a while, changing that behaviour seems
acceptable.

Would the same be true for changing "-feature,+feature"? (i.e. what this
patch does) Really: can this just be changed, or does there have to be
some period where the behaviour stays the same with a warning?

>> 
>> Signed-off-by: David Edmondson <david.edmondson@oracle.com>
>> ---
>>  target/i386/cpu.c                   | 33 +++++++++++++++++++++++------
>>  tests/qtest/test-x86-cpuid-compat.c |  8 +++----
>>  2 files changed, 30 insertions(+), 11 deletions(-)
>> 
>> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
>> index 35459a38bb..13f58ef183 100644
>> --- a/target/i386/cpu.c
>> +++ b/target/i386/cpu.c
>> @@ -4750,13 +4750,32 @@ static void x86_cpu_parse_featurestr(const char *typename, char *features,
>>          GlobalProperty *prop;
>>  
>>          /* Compatibility syntax: */
>> -        if (featurestr[0] == '+') {
>> -            plus_features = g_list_append(plus_features,
>> -                                          g_strdup(featurestr + 1));
>> -            continue;
>> -        } else if (featurestr[0] == '-') {
>> -            minus_features = g_list_append(minus_features,
>> -                                           g_strdup(featurestr + 1));
>> +        if (featurestr[0] == '+' || featurestr[0] == '-') {
>> +            const char *feat = featurestr + 1;
>> +            GList **remove, **add;
>> +            GList *val;
>> +
>> +            if (featurestr[0] == '+') {
>> +                remove = &minus_features;
>> +                add = &plus_features;
>> +            } else {
>> +                remove = &plus_features;
>> +                add = &minus_features;
>> +            }
>> +
>> +            val = g_list_find_custom(*remove, feat, compare_string);
>> +            if (val) {
>> +                char *data = val->data;
>> +
>> +                *remove = g_list_remove(*remove, data);
>> +                g_free(data);
>> +            }
>> +
>> +            val = g_list_find_custom(*add, feat, compare_string);
>> +            if (!val) {
>> +                *add = g_list_append(*add, g_strdup(feat));
>> +            }
>
> I believe we'll be able to get rid of plus_features/minus_features
> completely if we remove compatibility of "-feature,feature=on".
> But if we don't, wouldn't it be simpler to replace
> plus_features/minus_features with a single list, appending items
> in the order they appear?

Will investigate.

>> +
>>              continue;
>>          }
>>  
>> diff --git a/tests/qtest/test-x86-cpuid-compat.c b/tests/qtest/test-x86-cpuid-compat.c
>> index 7ca1883a29..6824d2b13e 100644
>> --- a/tests/qtest/test-x86-cpuid-compat.c
>> +++ b/tests/qtest/test-x86-cpuid-compat.c
>> @@ -171,18 +171,18 @@ static void test_plus_minus_subprocess(void)
>>      char *path;
>>  
>>      /* Rules:
>> -     * 1)"-foo" overrides "+foo"
>> +     * 1) The later of "+foo" or "-foo" wins
>>       * 2) "[+-]foo" overrides "foo=..."
>>       * 3) Old feature names with underscores (e.g. "sse4_2")
>>       *    should keep working
>>       *
>> -     * Note: rules 1 and 2 are planned to be removed soon, and
>> -     * should generate a warning.
>> +     * Note: rule 2 is planned to be removed soon, and should generate
>> +     * a warning.
>>       */
>>      qtest_start("-cpu pentium,-fpu,+fpu,-mce,mce=on,+cx8,cx8=off,+sse4_1,sse4_2=on");
>>      path = get_cpu0_qom_path();
>>  
>> -    g_assert_false(qom_get_bool(path, "fpu"));
>> +    g_assert_true(qom_get_bool(path, "fpu"));
>>      g_assert_false(qom_get_bool(path, "mce"));
>>      g_assert_true(qom_get_bool(path, "cx8"));
>>  
>> -- 
>> 2.29.2
>> 
>
> -- 
> Eduardo

dme.
-- 
They must have taken my marbles away.

On Tue, Jan 19, 2021 at 04:27:56PM +0000, David Edmondson wrote:
> On Tuesday, 2021-01-19 at 10:20:56 -05, Eduardo Habkost wrote:
> 
> > Hi,
> >
> > Thanks for the patch.  Getting rid of special -feature/+feature
> > behavior was in our TODO list for a long time.
> >
> > On Tue, Jan 19, 2021 at 02:22:06PM +0000, David Edmondson wrote:
> >> "Minus" features are applied after "plus" features, so ensure that a
> >> later "plus" feature causes an earlier "minus" feature to be removed.
> >> 
> >> This has no effect on the existing "-feature,feature=on" backward
> >> compatibility code (which warns and turns the feature off).
> >
> > If we are changing behavior, why not change behavior of
> > "-feature,feature=on" at the same time?  This would allow us to
> > get rid of plus_features/minus_features completely and just make
> > +feature/-feature synonyms to feature=on/feature=off.
> 
> Okay, I'll do that.
> 
> Given that there have been warnings associated with
> "-feature,feature=on" for a while, changing that behaviour seems
> acceptable.
> 
> Would the same be true for changing "-feature,+feature"? (i.e. what this
> patch does) Really: can this just be changed, or does there have to be
> some period where the behaviour stays the same with a warning?

I actually expected warnings to be triggered when using
"-feature,+feature" as well.  If we were not generating warnings
for that case, it will need more careful evaluation, just to be
sure it's safe.  Igor, do you remember the details here?

-- 
Eduardo

On Tue, 19 Jan 2021 11:30:52 -0500
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Tue, Jan 19, 2021 at 04:27:56PM +0000, David Edmondson wrote:
> > On Tuesday, 2021-01-19 at 10:20:56 -05, Eduardo Habkost wrote:
> >   
> > > Hi,
> > >
> > > Thanks for the patch.  Getting rid of special -feature/+feature
> > > behavior was in our TODO list for a long time.
> > >
> > > On Tue, Jan 19, 2021 at 02:22:06PM +0000, David Edmondson wrote:  
> > >> "Minus" features are applied after "plus" features, so ensure that a
> > >> later "plus" feature causes an earlier "minus" feature to be removed.
> > >> 
> > >> This has no effect on the existing "-feature,feature=on" backward
> > >> compatibility code (which warns and turns the feature off).  
> > >
> > > If we are changing behavior, why not change behavior of
> > > "-feature,feature=on" at the same time?  This would allow us to
> > > get rid of plus_features/minus_features completely and just make
> > > +feature/-feature synonyms to feature=on/feature=off.  
> > 
> > Okay, I'll do that.
> > 
> > Given that there have been warnings associated with
> > "-feature,feature=on" for a while, changing that behaviour seems
> > acceptable.
> > 
> > Would the same be true for changing "-feature,+feature"? (i.e. what this
> > patch does) Really: can this just be changed, or does there have to be
> > some period where the behaviour stays the same with a warning?  
> 
> I actually expected warnings to be triggered when using
> "-feature,+feature" as well.  If we were not generating warnings
> for that case, it will need more careful evaluation, just to be
> sure it's safe.  Igor, do you remember the details here?
As part of preparation to define/create machines via QMP,
I plan to post patch(s) to deprecate +-features in 6.0
(including special casing for -feat behavior (affects x86/sparc only))
and drop support for +-feat in 2 releases.
So we should end up with canonical property behavior only like all other
CPUs and devices.

On Wednesday, 2021-01-20 at 10:59:24 +01, Igor Mammedov wrote:

> On Tue, 19 Jan 2021 11:30:52 -0500
> Eduardo Habkost <ehabkost@redhat.com> wrote:
>
>> On Tue, Jan 19, 2021 at 04:27:56PM +0000, David Edmondson wrote:
>> > On Tuesday, 2021-01-19 at 10:20:56 -05, Eduardo Habkost wrote:
>> >   
>> > > Hi,
>> > >
>> > > Thanks for the patch.  Getting rid of special -feature/+feature
>> > > behavior was in our TODO list for a long time.
>> > >
>> > > On Tue, Jan 19, 2021 at 02:22:06PM +0000, David Edmondson wrote:  
>> > >> "Minus" features are applied after "plus" features, so ensure that a
>> > >> later "plus" feature causes an earlier "minus" feature to be removed.
>> > >> 
>> > >> This has no effect on the existing "-feature,feature=on" backward
>> > >> compatibility code (which warns and turns the feature off).  
>> > >
>> > > If we are changing behavior, why not change behavior of
>> > > "-feature,feature=on" at the same time?  This would allow us to
>> > > get rid of plus_features/minus_features completely and just make
>> > > +feature/-feature synonyms to feature=on/feature=off.  
>> > 
>> > Okay, I'll do that.
>> > 
>> > Given that there have been warnings associated with
>> > "-feature,feature=on" for a while, changing that behaviour seems
>> > acceptable.
>> > 
>> > Would the same be true for changing "-feature,+feature"? (i.e. what this
>> > patch does) Really: can this just be changed, or does there have to be
>> > some period where the behaviour stays the same with a warning?  
>> 
>> I actually expected warnings to be triggered when using
>> "-feature,+feature" as well.  If we were not generating warnings
>> for that case, it will need more careful evaluation, just to be
>> sure it's safe.  Igor, do you remember the details here?
> As part of preparation to define/create machines via QMP,
> I plan to post patch(s) to deprecate +-features in 6.0
> (including special casing for -feat behavior (affects x86/sparc only))
> and drop support for +-feat in 2 releases.
> So we should end up with canonical property behavior only like all other
> CPUs and devices.

In that case I will abandon this change and focus on getting my upstack
consumer to switch away from using +-.

Thanks.

dme.
-- 
Sometimes these eyes, forget the face they're peering from.

On Tue, Jan 19, 2021 at 11:30:52AM -0500, Eduardo Habkost wrote:
> On Tue, Jan 19, 2021 at 04:27:56PM +0000, David Edmondson wrote:
> > On Tuesday, 2021-01-19 at 10:20:56 -05, Eduardo Habkost wrote:
> > 
> > > Hi,
> > >
> > > Thanks for the patch.  Getting rid of special -feature/+feature
> > > behavior was in our TODO list for a long time.
> > >
> > > On Tue, Jan 19, 2021 at 02:22:06PM +0000, David Edmondson wrote:
> > >> "Minus" features are applied after "plus" features, so ensure that a
> > >> later "plus" feature causes an earlier "minus" feature to be removed.
> > >> 
> > >> This has no effect on the existing "-feature,feature=on" backward
> > >> compatibility code (which warns and turns the feature off).
> > >
> > > If we are changing behavior, why not change behavior of
> > > "-feature,feature=on" at the same time?  This would allow us to
> > > get rid of plus_features/minus_features completely and just make
> > > +feature/-feature synonyms to feature=on/feature=off.
> > 
> > Okay, I'll do that.
> > 
> > Given that there have been warnings associated with
> > "-feature,feature=on" for a while, changing that behaviour seems
> > acceptable.
> > 
> > Would the same be true for changing "-feature,+feature"? (i.e. what this
> > patch does) Really: can this just be changed, or does there have to be
> > some period where the behaviour stays the same with a warning?
> 
> I actually expected warnings to be triggered when using
> "-feature,+feature" as well.  If we were not generating warnings
> for that case, it will need more careful evaluation, just to be
> sure it's safe.  Igor, do you remember the details here?

Where are you expecting warnings ? I don't see any when launching QEMU

$ qemu-system-x86_64 -display none  -cpu Westmere,-vmx 

$ qemu-system-x86_64 -display none  -cpu Westmere,-vmx,sse=on

neither produces warnings, even with current git master.

I don't think we can change the parsing behaviour here without impacting
guest ABI, and that feels dangerous given that we've not been warning
people the syntax is undesirable.

IMHO just leave the parsing unchanged, deprecate it, and then delete
the code.  We don't need to "improve" usability semantics of something
that we want to delete anyway.

Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|

On Wednesday, 2021-01-20 at 10:08:03 GMT, Daniel P. Berrangé wrote:

> On Tue, Jan 19, 2021 at 11:30:52AM -0500, Eduardo Habkost wrote:
>> On Tue, Jan 19, 2021 at 04:27:56PM +0000, David Edmondson wrote:
>> > On Tuesday, 2021-01-19 at 10:20:56 -05, Eduardo Habkost wrote:
>> > 
>> > > Hi,
>> > >
>> > > Thanks for the patch.  Getting rid of special -feature/+feature
>> > > behavior was in our TODO list for a long time.
>> > >
>> > > On Tue, Jan 19, 2021 at 02:22:06PM +0000, David Edmondson wrote:
>> > >> "Minus" features are applied after "plus" features, so ensure that a
>> > >> later "plus" feature causes an earlier "minus" feature to be removed.
>> > >> 
>> > >> This has no effect on the existing "-feature,feature=on" backward
>> > >> compatibility code (which warns and turns the feature off).
>> > >
>> > > If we are changing behavior, why not change behavior of
>> > > "-feature,feature=on" at the same time?  This would allow us to
>> > > get rid of plus_features/minus_features completely and just make
>> > > +feature/-feature synonyms to feature=on/feature=off.
>> > 
>> > Okay, I'll do that.
>> > 
>> > Given that there have been warnings associated with
>> > "-feature,feature=on" for a while, changing that behaviour seems
>> > acceptable.
>> > 
>> > Would the same be true for changing "-feature,+feature"? (i.e. what this
>> > patch does) Really: can this just be changed, or does there have to be
>> > some period where the behaviour stays the same with a warning?
>> 
>> I actually expected warnings to be triggered when using
>> "-feature,+feature" as well.  If we were not generating warnings
>> for that case, it will need more careful evaluation, just to be
>> sure it's safe.  Igor, do you remember the details here?
>
> Where are you expecting warnings ? I don't see any when launching QEMU

qemu-system-x86_64 -display none -cpu Westmere,-vmx,+vmx

Warnings because the result of this is "-vmx".

> IMHO just leave the parsing unchanged, deprecate it, and then delete
> the code.  We don't need to "improve" usability semantics of something
> that we want to delete anyway.

/me nods.

dme.
-- 
I just bite it, it's for the look I don't light it.

On Wed, Jan 20, 2021 at 10:17:36AM +0000, David Edmondson wrote:
> On Wednesday, 2021-01-20 at 10:08:03 GMT, Daniel P. Berrangé wrote:
> 
> > On Tue, Jan 19, 2021 at 11:30:52AM -0500, Eduardo Habkost wrote:
> >> On Tue, Jan 19, 2021 at 04:27:56PM +0000, David Edmondson wrote:
> >> > On Tuesday, 2021-01-19 at 10:20:56 -05, Eduardo Habkost wrote:
> >> > 
> >> > > Hi,
> >> > >
> >> > > Thanks for the patch.  Getting rid of special -feature/+feature
> >> > > behavior was in our TODO list for a long time.
> >> > >
> >> > > On Tue, Jan 19, 2021 at 02:22:06PM +0000, David Edmondson wrote:
> >> > >> "Minus" features are applied after "plus" features, so ensure that a
> >> > >> later "plus" feature causes an earlier "minus" feature to be removed.
> >> > >> 
> >> > >> This has no effect on the existing "-feature,feature=on" backward
> >> > >> compatibility code (which warns and turns the feature off).
> >> > >
> >> > > If we are changing behavior, why not change behavior of
> >> > > "-feature,feature=on" at the same time?  This would allow us to
> >> > > get rid of plus_features/minus_features completely and just make
> >> > > +feature/-feature synonyms to feature=on/feature=off.
> >> > 
> >> > Okay, I'll do that.
> >> > 
> >> > Given that there have been warnings associated with
> >> > "-feature,feature=on" for a while, changing that behaviour seems
> >> > acceptable.
> >> > 
> >> > Would the same be true for changing "-feature,+feature"? (i.e. what this
> >> > patch does) Really: can this just be changed, or does there have to be
> >> > some period where the behaviour stays the same with a warning?
> >> 
> >> I actually expected warnings to be triggered when using
> >> "-feature,+feature" as well.  If we were not generating warnings
> >> for that case, it will need more careful evaluation, just to be
> >> sure it's safe.  Igor, do you remember the details here?
> >
> > Where are you expecting warnings ? I don't see any when launching QEMU
> 
> qemu-system-x86_64 -display none -cpu Westmere,-vmx,+vmx
> 
> Warnings because the result of this is "-vmx".
> 
> > IMHO just leave the parsing unchanged, deprecate it, and then delete
> > the code.  We don't need to "improve" usability semantics of something
> > that we want to delete anyway.
> 
> /me nods.

I agree, but I guess we need to convince Paolo:
https://lore.kernel.org/qemu-devel/1990888058.22417362.1465939000140.JavaMail.zimbra@redhat.com/

-- 
Eduardo

On Wed, 20 Jan 2021 11:18:01 -0500
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Wed, Jan 20, 2021 at 10:17:36AM +0000, David Edmondson wrote:
> > On Wednesday, 2021-01-20 at 10:08:03 GMT, Daniel P. Berrangé wrote:
> >   
> > > On Tue, Jan 19, 2021 at 11:30:52AM -0500, Eduardo Habkost wrote:  
> > >> On Tue, Jan 19, 2021 at 04:27:56PM +0000, David Edmondson wrote:  
> > >> > On Tuesday, 2021-01-19 at 10:20:56 -05, Eduardo Habkost wrote:
> > >> >   
> > >> > > Hi,
> > >> > >
> > >> > > Thanks for the patch.  Getting rid of special -feature/+feature
> > >> > > behavior was in our TODO list for a long time.
> > >> > >
> > >> > > On Tue, Jan 19, 2021 at 02:22:06PM +0000, David Edmondson wrote:  
> > >> > >> "Minus" features are applied after "plus" features, so ensure that a
> > >> > >> later "plus" feature causes an earlier "minus" feature to be removed.
> > >> > >> 
> > >> > >> This has no effect on the existing "-feature,feature=on" backward
> > >> > >> compatibility code (which warns and turns the feature off).  
> > >> > >
> > >> > > If we are changing behavior, why not change behavior of
> > >> > > "-feature,feature=on" at the same time?  This would allow us to
> > >> > > get rid of plus_features/minus_features completely and just make
> > >> > > +feature/-feature synonyms to feature=on/feature=off.  
> > >> > 
> > >> > Okay, I'll do that.
> > >> > 
> > >> > Given that there have been warnings associated with
> > >> > "-feature,feature=on" for a while, changing that behaviour seems
> > >> > acceptable.
> > >> > 
> > >> > Would the same be true for changing "-feature,+feature"? (i.e. what this
> > >> > patch does) Really: can this just be changed, or does there have to be
> > >> > some period where the behaviour stays the same with a warning?  
> > >> 
> > >> I actually expected warnings to be triggered when using
> > >> "-feature,+feature" as well.  If we were not generating warnings
> > >> for that case, it will need more careful evaluation, just to be
> > >> sure it's safe.  Igor, do you remember the details here?  
> > >
> > > Where are you expecting warnings ? I don't see any when launching QEMU  
> > 
> > qemu-system-x86_64 -display none -cpu Westmere,-vmx,+vmx
> > 
> > Warnings because the result of this is "-vmx".
> >   
> > > IMHO just leave the parsing unchanged, deprecate it, and then delete
> > > the code.  We don't need to "improve" usability semantics of something
> > > that we want to delete anyway.  
> > 
> > /me nods.  
> 
> I agree, but I guess we need to convince Paolo:
> https://lore.kernel.org/qemu-devel/1990888058.22417362.1465939000140.JavaMail.zimbra@redhat.com/
that's ancient :)

He recently started this revolution himself :)
https://www.mail-archive.com/qemu-devel@nongnu.org/msg757280.html

That's why I have -cpu +/-foo deprecation on my not too far away TODO list.

The ordering semantics of +feature/-feature is tricky and not
obvious, and it requires a custom option parser.  Deprecate that
syntax so we can eventually remove the custom -cpu option parser
and plus_features/minus_features global variables in i386.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
 docs/system/deprecated.rst | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst
index e20bfcb17a4..2c4b8d4b78b 100644
--- a/docs/system/deprecated.rst
+++ b/docs/system/deprecated.rst
@@ -127,6 +127,20 @@ Drives with interface types other than ``if=none`` are for onboard
 devices.  It is possible to use drives the board doesn't pick up with
 -device.  This usage is now deprecated.  Use ``if=none`` instead.
 
+``-cpu`` ``+feature`` and ``-feature`` syntax (since 6.0.0)
+'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
+
+The ``-cpu ...,+feature`` and ``-cpu ...,-feature`` syntax for
+enabling and disabling CPU features is deprecated.  The ``-cpu
+...,feature=on`` or ``-cpu ...,feature=off`` should be used
+instead.
+
+Note that the ordering semantics of ``-cpu ...,-feature,+feature``
+is different from ``-cpu ...,feature=off,feature=on``.  With the
+former, the feature got disabled because ``-feature`` had
+precedence, but with the latter the feature will be enabled
+because options are applied in the order they appear.
+
 
 QEMU Machine Protocol (QMP) commands
 ------------------------------------
-- 
2.28.0

On Wednesday, 2021-01-20 at 15:12:41 -05, Eduardo Habkost wrote:

> The ordering semantics of +feature/-feature is tricky and not
> obvious, and it requires a custom option parser.  Deprecate that
> syntax so we can eventually remove the custom -cpu option parser
> and plus_features/minus_features global variables in i386.
>
> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

With very minor wording comment...

Reviewed-by: David Edmondson <david.edmondson@oracle.com>

> ---
>  docs/system/deprecated.rst | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst
> index e20bfcb17a4..2c4b8d4b78b 100644
> --- a/docs/system/deprecated.rst
> +++ b/docs/system/deprecated.rst
> @@ -127,6 +127,20 @@ Drives with interface types other than ``if=none`` are for onboard
>  devices.  It is possible to use drives the board doesn't pick up with
>  -device.  This usage is now deprecated.  Use ``if=none`` instead.
>  
> +``-cpu`` ``+feature`` and ``-feature`` syntax (since 6.0.0)
> +'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
> +
> +The ``-cpu ...,+feature`` and ``-cpu ...,-feature`` syntax for
> +enabling and disabling CPU features is deprecated.  The ``-cpu
> +...,feature=on`` or ``-cpu ...,feature=off`` should be used
> +instead.
> +
> +Note that the ordering semantics of ``-cpu ...,-feature,+feature``
> +is different from ``-cpu ...,feature=off,feature=on``.  With the
> +former, the feature got disabled because ``-feature`` had

s/got/was/

> +precedence, but with the latter the feature will be enabled
> +because options are applied in the order they appear.
> +
>  
>  QEMU Machine Protocol (QMP) commands
>  ------------------------------------
> -- 
> 2.28.0

dme.
-- 
I got a girlfriend that's better than that.

On Wed, Jan 20, 2021 at 03:12:41PM -0500, Eduardo Habkost wrote:
> The ordering semantics of +feature/-feature is tricky and not
> obvious, and it requires a custom option parser.  Deprecate that
> syntax so we can eventually remove the custom -cpu option parser
> and plus_features/minus_features global variables in i386.
> 
> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> ---
>  docs/system/deprecated.rst | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)

Ideally we would also print a warning on stderr when this deprecated
style is used.

> 
> diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst
> index e20bfcb17a4..2c4b8d4b78b 100644
> --- a/docs/system/deprecated.rst
> +++ b/docs/system/deprecated.rst
> @@ -127,6 +127,20 @@ Drives with interface types other than ``if=none`` are for onboard
>  devices.  It is possible to use drives the board doesn't pick up with
>  -device.  This usage is now deprecated.  Use ``if=none`` instead.
>  
> +``-cpu`` ``+feature`` and ``-feature`` syntax (since 6.0.0)
> +'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
> +
> +The ``-cpu ...,+feature`` and ``-cpu ...,-feature`` syntax for
> +enabling and disabling CPU features is deprecated.  The ``-cpu
> +...,feature=on`` or ``-cpu ...,feature=off`` should be used
> +instead.
> +
> +Note that the ordering semantics of ``-cpu ...,-feature,+feature``
> +is different from ``-cpu ...,feature=off,feature=on``.  With the
> +former, the feature got disabled because ``-feature`` had
> +precedence, but with the latter the feature will be enabled
> +because options are applied in the order they appear.
> +

Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|

On 1/21/21 4:39 AM, Daniel P. Berrangé wrote:
> On Wed, Jan 20, 2021 at 03:12:41PM -0500, Eduardo Habkost wrote:
>> The ordering semantics of +feature/-feature is tricky and not
>> obvious, and it requires a custom option parser.  Deprecate that
>> syntax so we can eventually remove the custom -cpu option parser
>> and plus_features/minus_features global variables in i386.
>>
>> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
>> ---
>>   docs/system/deprecated.rst | 14 ++++++++++++++
>>   1 file changed, 14 insertions(+)
> 
> Ideally we would also print a warning on stderr when this deprecated
> style is used.
> 

+1.

It might break some tests to do that, though, but if it's not too 
gruesome it's probably worth it.

On Wed, 20 Jan 2021 15:12:41 -0500
Eduardo Habkost <ehabkost@redhat.com> wrote:

> The ordering semantics of +feature/-feature is tricky and not
> obvious, and it requires a custom option parser.  Deprecate that
> syntax so we can eventually remove the custom -cpu option parser
> and plus_features/minus_features global variables in i386.
it affects spark as well

with that

Reviewed-by: Igor Mammedov <imammedo@redhat.com>

> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> ---
>  docs/system/deprecated.rst | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst
> index e20bfcb17a4..2c4b8d4b78b 100644
> --- a/docs/system/deprecated.rst
> +++ b/docs/system/deprecated.rst
> @@ -127,6 +127,20 @@ Drives with interface types other than ``if=none`` are for onboard
>  devices.  It is possible to use drives the board doesn't pick up with
>  -device.  This usage is now deprecated.  Use ``if=none`` instead.
>  
> +``-cpu`` ``+feature`` and ``-feature`` syntax (since 6.0.0)
> +'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
> +
> +The ``-cpu ...,+feature`` and ``-cpu ...,-feature`` syntax for
> +enabling and disabling CPU features is deprecated.  The ``-cpu
> +...,feature=on`` or ``-cpu ...,feature=off`` should be used
> +instead.
> +
> +Note that the ordering semantics of ``-cpu ...,-feature,+feature``
> +is different from ``-cpu ...,feature=off,feature=on``.  With the
> +former, the feature got disabled because ``-feature`` had
> +precedence, but with the latter the feature will be enabled
> +because options are applied in the order they appear.
> +
>  
>  QEMU Machine Protocol (QMP) commands
>  ------------------------------------