exec: Skip mru section if it's a partial page and not resolving subpage

[Qemu-devel] [PATCH] exec: Skip mru section if it's a partial page and not resolving subpage

Posted by BALATON Zoltan 6 years, 5 months ago

This fixes a crash caused by picking the wrong memory region in
address_space_lookup_region seen with client code accessing a device
model that uses alias memory regions.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
---
 exec.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/exec.c b/exec.c
index 97a24a8..e5f2b9a 100644
--- a/exec.c
+++ b/exec.c
@@ -413,6 +413,7 @@ static MemoryRegionSection *address_space_lookup_region(AddressSpaceDispatch *d,
     bool update;
 
     if (section && section != &d->map.sections[PHYS_SECTION_UNASSIGNED] &&
+        (resolve_subpage || !section->offset_within_region) &&
         section_covers_addr(section, addr)) {
         update = false;
     } else {
-- 
2.7.6

Re: [Qemu-devel] [PATCH] exec: Skip mru section if it's a partial page and not resolving subpage

Posted by Paolo Bonzini 6 years, 5 months ago

On 14/11/2017 23:42, BALATON Zoltan wrote:
> This fixes a crash caused by picking the wrong memory region in
> address_space_lookup_region seen with client code accessing a device
> model that uses alias memory regions.
> 
> Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
> ---
>  exec.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/exec.c b/exec.c
> index 97a24a8..e5f2b9a 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -413,6 +413,7 @@ static MemoryRegionSection *address_space_lookup_region(AddressSpaceDispatch *d,
>      bool update;
>  
>      if (section && section != &d->map.sections[PHYS_SECTION_UNASSIGNED] &&
> +        (resolve_subpage || !section->offset_within_region) &&
>          section_covers_addr(section, addr)) {
>          update = false;
>      } else {
> 

This is another possibility:

diff --git a/exec.c b/exec.c
index 97a24a875e..3bb9fcf257 100644
--- a/exec.c
+++ b/exec.c
@@ -410,22 +410,16 @@ static MemoryRegionSection *address_space_lookup_region(AddressSpaceDispatch *d,
 {
     MemoryRegionSection *section = atomic_read(&d->mru_section);
     subpage_t *subpage;
-    bool update;
 
-    if (section && section != &d->map.sections[PHYS_SECTION_UNASSIGNED] &&
-        section_covers_addr(section, addr)) {
-        update = false;
-    } else {
+    if (!section || section == &d->map.sections[PHYS_SECTION_UNASSIGNED] ||
+        !section_covers_addr(section, addr)) {
         section = phys_page_find(d, addr);
-        update = true;
+        atomic_set(&d->mru_section, section);
     }
     if (resolve_subpage && section->mr->subpage) {
         subpage = container_of(section->mr, subpage_t, iomem);
         section = &d->map.sections[subpage->sub_section[SUBPAGE_IDX(addr)]];
     }
-    if (update) {
-        atomic_set(&d->mru_section, section);
-    }
     return section;
 }
 

It will skip the expensive phys_page_find but not the cheap subpage lookup.
Does it work for you?

Paolo

Re: [Qemu-devel] [PATCH] exec: Skip mru section if it's a partial page and not resolving subpage

Posted by BALATON Zoltan 6 years, 5 months ago

On Wed, 15 Nov 2017, Paolo Bonzini wrote:
> This is another possibility:
>
> diff --git a/exec.c b/exec.c
> index 97a24a875e..3bb9fcf257 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -410,22 +410,16 @@ static MemoryRegionSection *address_space_lookup_region(AddressSpaceDispatch *d,
> {
>     MemoryRegionSection *section = atomic_read(&d->mru_section);
>     subpage_t *subpage;
> -    bool update;
>
> -    if (section && section != &d->map.sections[PHYS_SECTION_UNASSIGNED] &&
> -        section_covers_addr(section, addr)) {
> -        update = false;
> -    } else {
> +    if (!section || section == &d->map.sections[PHYS_SECTION_UNASSIGNED] ||
> +        !section_covers_addr(section, addr)) {
>         section = phys_page_find(d, addr);
> -        update = true;
> +        atomic_set(&d->mru_section, section);
>     }
>     if (resolve_subpage && section->mr->subpage) {
>         subpage = container_of(section->mr, subpage_t, iomem);
>         section = &d->map.sections[subpage->sub_section[SUBPAGE_IDX(addr)]];
>     }
> -    if (update) {
> -        atomic_set(&d->mru_section, section);
> -    }
>     return section;
> }
>
>
> It will skip the expensive phys_page_find but not the cheap subpage lookup.
> Does it work for you?

This also works but I can't understand why becuase the problematic call to 
this function which got the wrong result from section_covers_addr was with 
resolve_subpage=false and in that case this looks identical to the 
original which did not work. Unless this function is called multiple times 
and another call with resolve_subpage=true now somehow works better with 
this change or replacing the mru_section earlier before subpage lookup 
makes a difference which is not obvious from this function.

Anyway, it fixes the problem I see so you can use this instead of my 
patch.

Thank you,
BALATON Zoltan

Re: [Qemu-devel] [PATCH] exec: Skip mru section if it's a partial page and not resolving subpage

Posted by Paolo Bonzini 6 years, 5 months ago

On 15/11/2017 20:00, BALATON Zoltan wrote:
> On Wed, 15 Nov 2017, Paolo Bonzini wrote:
>> This is another possibility:
>>
>> diff --git a/exec.c b/exec.c
>> index 97a24a875e..3bb9fcf257 100644
>> --- a/exec.c
>> +++ b/exec.c
>> @@ -410,22 +410,16 @@ static MemoryRegionSection
>> *address_space_lookup_region(AddressSpaceDispatch *d,
>> {
>>     MemoryRegionSection *section = atomic_read(&d->mru_section);
>>     subpage_t *subpage;
>> -    bool update;
>>
>> -    if (section && section !=
>> &d->map.sections[PHYS_SECTION_UNASSIGNED] &&
>> -        section_covers_addr(section, addr)) {
>> -        update = false;
>> -    } else {
>> +    if (!section || section ==
>> &d->map.sections[PHYS_SECTION_UNASSIGNED] ||
>> +        !section_covers_addr(section, addr)) {
>>         section = phys_page_find(d, addr);
>> -        update = true;
>> +        atomic_set(&d->mru_section, section);
>>     }
>>     if (resolve_subpage && section->mr->subpage) {
>>         subpage = container_of(section->mr, subpage_t, iomem);
>>         section =
>> &d->map.sections[subpage->sub_section[SUBPAGE_IDX(addr)]];
>>     }
>> -    if (update) {
>> -        atomic_set(&d->mru_section, section);
>> -    }
>>     return section;
>> }
>>
>>
>> It will skip the expensive phys_page_find but not the cheap subpage
>> lookup.
>> Does it work for you?
> 
> This also works but I can't understand why becuase the problematic call
> to this function which got the wrong result from section_covers_addr was
> with resolve_subpage=false and in that case this looks identical to the
> original which did not work.

The problem was that a resolve_subpage=true mru_section was reused for a
resolve_subpage=false mru_section.  This is fixed by always making the
mru_section the same as if resolve_subpage=false (mru_section is set
before looking into the subpage_t).

Thanks,

Paolo

> Unless this function is called multiple
> times and another call with resolve_subpage=true now somehow works
> better with this change or replacing the mru_section earlier before
> subpage lookup makes a difference which is not obvious from this function.
> 
> Anyway, it fixes the problem I see so you can use this instead of my patch.
> 
> Thank you,
> BALATON Zoltan