Re: [PATCH 2/2] i386/cpu: Mask off unsupported XSAVE components

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 7/16/2020 11:14 PM, Eduardo Habkost wrote:
> On Thu, Jul 16, 2020 at 04:20:19PM +0800, Xiaoyao Li wrote:
>> When setting up XSAVE components, it needs to mask off those unsupported
>> by KVM.
>>
>> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
> 
> We must never disable CPUID features silently based on host
> capabilities, otherwise we can't guarantee guest ABI stability
> when migrating to another host.  Filtering of features should
> involve a call to mark_unavailable_features() (or some equivalent
> mechanism) so we can report the missing features properly through
> QMP.
> 
> Could you explain what's the bug you are trying to fix?  The loop
> at x86_cpu_filter_features() is already supposed to disable
> features unsupported by the host.

Sorry, I forgot x86_cpu_filter_features() totally when code inspection.

>> ---
>>   target/i386/cpu.c | 6 ++++--
>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
>> index f5f11603e805..efc92334b7b1 100644
>> --- a/target/i386/cpu.c
>> +++ b/target/i386/cpu.c
>> @@ -6274,8 +6274,10 @@ static void x86_cpu_enable_xsave_components(X86CPU *cpu)
>>           }
>>       }
>>   
>> -    env->features[FEAT_XSAVE_COMP_LO] = mask;
>> -    env->features[FEAT_XSAVE_COMP_HI] = mask >> 32;
>> +    env->features[FEAT_XSAVE_COMP_LO] = mask &
>> +        x86_cpu_get_supported_feature_word(FEAT_XSAVE_COMP_LO, cpu->migratable);
>> +    env->features[FEAT_XSAVE_COMP_HI] = (mask >> 32) &
>> +        x86_cpu_get_supported_feature_word(FEAT_XSAVE_COMP_HI, cpu->migratable);
>>   }
>>   
>>   /***** Steps involved on loading and filtering CPUID data
>> -- 
>> 2.18.4
>>
>