Re: [Xen-devel] [PATCH v2 1/2] xc_cpuid_x86.c: Simplify masking conditions and remove redundant work

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-09-09 at 11:45 +0100, Jan Beulich wrote:
> >>> On 09.09.14 at 06:31, <alfred.z.song@xxxxxxxxx> wrote:
> > @@ -195,16 +186,14 @@ static void intel_xc_cpuid_policy(
> >          break;
> >  
> >      case 0x80000001: {
> > -        int is_64bit = hypervisor_is_64bit(xch) && is_pae;
> > -
> >          /* Only a few features are advertised in Intel's 0x80000001. */
> > -        regs[2] &= (is_64bit ? bitmaskof(X86_FEATURE_LAHF_LM) : 0) |
> > -                               bitmaskof(X86_FEATURE_3DNOWPREFETCH) |
> > -                               bitmaskof(X86_FEATURE_ABM);
> > -        regs[3] &= ((is_pae ? bitmaskof(X86_FEATURE_NX) : 0) |
> > -                    (is_64bit ? bitmaskof(X86_FEATURE_LM) : 0) |
> > -                    (is_64bit ? bitmaskof(X86_FEATURE_SYSCALL) : 0) |
> > -                    (is_64bit ? bitmaskof(X86_FEATURE_RDTSCP) : 0));
> > +        regs[2] &= (bitmaskof(X86_FEATURE_LAHF_LM) |
> > +                    bitmaskof(X86_FEATURE_3DNOWPREFETCH) |
> > +                    bitmaskof(X86_FEATURE_ABM);
> > +        regs[3] &= (bitmaskof(X86_FEATURE_NX) |
> > +                    bitmaskof(X86_FEATURE_LM) |
> > +                    (is_pae ? bitmaskof(X86_FEATURE_SYSCALL) : 0) |
> > +                    (is_pae ? bitmaskof(X86_FEATURE_RDTSCP) : 0));
> 
> As said before, tying these two features to is_pae seems a
> little strange, but if the tools maintainers can live with that, I
> guess I can too (short of having a better suggestion other
> than to drop the conditionals altogether).

Patch #2 here seems to remove it from the RDTSCP, surely that should be
folded in.

I also don't understand the link between PAE and the presence of
SYSCALL.

Ian.


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