We ignore silently the value of smp_threads when we set
the VSMT value, and if smp_threads is greater than VSMT
kernel is going into trouble later.

Fixes: 8904e5a750
("spapr: Adjust default VSMT value for better migration compatibility")

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 hw/ppc/spapr.c | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index 32a876be56..638b3cafd0 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -2294,15 +2294,7 @@ static void spapr_set_vsmt_mode(sPAPRMachineState *spapr, Error **errp)
     }
 
     /* Detemine the VSMT mode to use: */
-    if (vsmt_user) {
-        if (spapr->vsmt < smp_threads) {
-            error_setg(&local_err, "Cannot support VSMT mode %d"
-                         " because it must be >= threads/core (%d)",
-                         spapr->vsmt, smp_threads);
-            goto out;
-        }
-        /* In this case, spapr->vsmt has been set by the command line */
-    } else {
+    if (!vsmt_user) {
         /*
          * Default VSMT value is tricky, because we need it to be as
          * consistent as possible (for migration), but this requires
@@ -2313,6 +2305,13 @@ static void spapr_set_vsmt_mode(sPAPRMachineState *spapr, Error **errp)
         spapr->vsmt = 8;
     }
 
+    if (spapr->vsmt < smp_threads) {
+        error_setg(&local_err, "Cannot support VSMT mode %d"
+                   " because it must be >= threads/core (%d)",
+                   spapr->vsmt, smp_threads);
+        goto out;
+    }
+
     /* KVM: If necessary, set the SMT mode: */
     if (kvm_enabled() && (spapr->vsmt != kvm_smt)) {
         ret = kvmppc_set_smt_threads(spapr->vsmt);
-- 
2.14.3

On Wed,  7 Feb 2018 17:17:30 +0100
Laurent Vivier <lvivier@redhat.com> wrote:

> We ignore silently the value of smp_threads when we set
> the VSMT value, and if smp_threads is greater than VSMT
> kernel is going into trouble later.
> 
> Fixes: 8904e5a750
> ("spapr: Adjust default VSMT value for better migration compatibility")
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  hw/ppc/spapr.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 32a876be56..638b3cafd0 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -2294,15 +2294,7 @@ static void spapr_set_vsmt_mode(sPAPRMachineState *spapr, Error **errp)
>      }
>  
>      /* Detemine the VSMT mode to use: */
> -    if (vsmt_user) {
> -        if (spapr->vsmt < smp_threads) {
> -            error_setg(&local_err, "Cannot support VSMT mode %d"
> -                         " because it must be >= threads/core (%d)",
> -                         spapr->vsmt, smp_threads);

In this case, we'd error out because the user requested two settings that
are known to be incompatible.

> -            goto out;
> -        }
> -        /* In this case, spapr->vsmt has been set by the command line */
> -    } else {
> +    if (!vsmt_user) {
>          /*
>           * Default VSMT value is tricky, because we need it to be as
>           * consistent as possible (for migration), but this requires
> @@ -2313,6 +2305,13 @@ static void spapr_set_vsmt_mode(sPAPRMachineState *spapr, Error **errp)
>          spapr->vsmt = 8;

In this case, we'd error out because the user requested a setting that is
incompatible with our default. Maybe worth a separate error message ?

"Cannot support %d threads/core because it must be <= to default VSMT mode (8)"

Anyway, the fix is good:

Reviewed-by: Greg Kurz <groug@kaod.org>

>      }
>  
> +    if (spapr->vsmt < smp_threads) {
> +        error_setg(&local_err, "Cannot support VSMT mode %d"
> +                   " because it must be >= threads/core (%d)",
> +                   spapr->vsmt, smp_threads);
> +        goto out;
> +    }
> +
>      /* KVM: If necessary, set the SMT mode: */
>      if (kvm_enabled() && (spapr->vsmt != kvm_smt)) {
>          ret = kvmppc_set_smt_threads(spapr->vsmt);

On 07/02/2018 21:21, Greg Kurz wrote:
> On Wed,  7 Feb 2018 17:17:30 +0100
> Laurent Vivier <lvivier@redhat.com> wrote:
> 
>> We ignore silently the value of smp_threads when we set
>> the VSMT value, and if smp_threads is greater than VSMT
>> kernel is going into trouble later.
>>
>> Fixes: 8904e5a750
>> ("spapr: Adjust default VSMT value for better migration compatibility")
>>
>> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
>> ---
>>  hw/ppc/spapr.c | 17 ++++++++---------
>>  1 file changed, 8 insertions(+), 9 deletions(-)
>>
>> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
>> index 32a876be56..638b3cafd0 100644
>> --- a/hw/ppc/spapr.c
>> +++ b/hw/ppc/spapr.c
>> @@ -2294,15 +2294,7 @@ static void spapr_set_vsmt_mode(sPAPRMachineState *spapr, Error **errp)
>>      }
>>  
>>      /* Detemine the VSMT mode to use: */
>> -    if (vsmt_user) {
>> -        if (spapr->vsmt < smp_threads) {
>> -            error_setg(&local_err, "Cannot support VSMT mode %d"
>> -                         " because it must be >= threads/core (%d)",
>> -                         spapr->vsmt, smp_threads);
> 
> In this case, we'd error out because the user requested two settings that
> are known to be incompatible.
> 
>> -            goto out;
>> -        }
>> -        /* In this case, spapr->vsmt has been set by the command line */
>> -    } else {
>> +    if (!vsmt_user) {
>>          /*
>>           * Default VSMT value is tricky, because we need it to be as
>>           * consistent as possible (for migration), but this requires
>> @@ -2313,6 +2305,13 @@ static void spapr_set_vsmt_mode(sPAPRMachineState *spapr, Error **errp)
>>          spapr->vsmt = 8;
> 
> In this case, we'd error out because the user requested a setting that is
> incompatible with our default. Maybe worth a separate error message ?
> 
> "Cannot support %d threads/core because it must be <= to default VSMT mode (8)"

I'm going to update the error message.

Thanks,
Laurent