Re: [Xen-devel] [PATCH] x86: clear CPUID output of leaf 0xd for Dom0 when xsave is disabled

[Wei Huang <wei.huang2@xxxxxxx>]

I tested cpuid on a real hardware. If software wants, reading all 
sub-leaves are allowed even on hardware which doesn't support XSAVE. The 
instruction just returns 0. So I don't think we need to zap output for 
sub-leaves > 1.
   0x0000000c 0x00: eax=0x00000000 ebx=0x00000000 ecx=0x00000000 
edx=0x00000000
   0x0000000d 0x00: eax=0x00000003 ebx=0x00000240 ecx=0x00000240 
edx=0x00000000
   0x0000000d 0x02: eax=0x00000000 ebx=0x00000000 ecx=0x00000000 
edx=0x00000000
   0x0000000d 0x3e: eax=0x00000000 ebx=0x00000000 ecx=0x00000000 
edx=0x00000000
On 05/18/2011 04:53 AM, Jan Beulich wrote:
> Linux starting with 2.6.36 uses the XSAVEOPT instruction and has
> certain code paths that look only at the feature bit reported through
> CPUID leaf 0xd sub-leaf 1 (i.e. without qualifying the check with one
> evaluating leaf 4 output). Consequently the hypervisor ought to mimic
> actual hardware in clearing leaf 0xd output when not supporting xsave.
>
> (Note that this is only a minimal fix. It may be necessary, e.g. for
> LWP, to also adjust sub-leaf 0's bit masks and perhaps zap output of
> sub-leaves>  1 when the respective bit in sub-leaf 0 is getting
> cleared.)
>
> Signed-off-by: Jan Beulich<jbeulich@xxxxxxxxxx>
>
> --- a/xen/arch/x86/traps.c
> +++ b/xen/arch/x86/traps.c
> @@ -836,6 +836,10 @@ static void pv_cpuid(struct cpu_user_reg
>           __clear_bit(X86_FEATURE_NODEID_MSR % 32,&c);
>           __clear_bit(X86_FEATURE_TOPOEXT % 32,&c);
>           break;
> +    case 0xd: /* XSAVE */
> +        if ( xsave_enabled(current) )
> +            break;
> +        /* fall through */
>       case 5: /* MONITOR/MWAIT */
>       case 0xa: /* Architectural Performance Monitor Features */
>       case 0x8000000a: /* SVM revision and features */

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