Re: [Xen-devel] [Patch] Allowing PV-OPS kernel to detect whether XSAVE is supported

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> On 09.11.10 at 11:43, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Tue, 2010-11-09 at 06:22 +0000, Haitao Shan wrote:
>> Hi, Jeremy,
>> 
>> This patch allows pv-ops kernel to detect whether XSAVE is supported
>> (before masking it unconditionally through xen_cpuid).
>> Can you please have review? Thanks!
>> 
>> Signed-off-by: Shan Haitao <haitao.shan@xxxxxxxxx>
>> 
>> Shan Haitao 
>> 
> 
>> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
>> index fd3803e..03bfaf7 100644
>> --- a/arch/x86/xen/enlighten.c
>> +++ b/arch/x86/xen/enlighten.c
>> @@ -252,6 +252,13 @@ static __init void xen_init_cpuid_mask(void)
>>                           (1 << X86_FEATURE_MCA)  |  /* disable MCA */
>>                           (1 << X86_FEATURE_APIC) |  /* disable local APIC */
>>                           (1 << X86_FEATURE_ACPI));  /* disable ACPI */
>> +       ax = 1;
>> +       xen_cpuid(&ax, &bx, &cx, &dx);
>> +
>> +       /* Xen will set CR4.OSXSAVE if supported and not disabled by force 
> */
> 
> For how long has the hypervisor had this behaviour? IIRC older
> hypervisors did not correctly expose/mask the *XSAVE CPUID flags and
> would causes PV guests to crash when they used *XSAVE features which
> weren't actually available.
> 
> In other words have you confirmed that this patch does not break the
> kernel running on older hypervisors such as Xen 4.0?

The problem was only with the XSAVE cpuid bit, not the OSXSAVE
one (which gets turned on only when CR4.OSXSAVE gets set).

Jan

>> +       if ( cx & (1 << (X86_FEATURE_XSAVE % 32)) &&
>> +               cx & (1 << (X86_FEATURE_OSXSAVE % 32)) )
>> +               return;
>>  
>>         cpuid_leaf1_ecx_mask &= ~(1 << (X86_FEATURE_XSAVE % 32)); /* disable 
> XSAVE */
>>  } 
> 
> 
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