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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.09.14 at 14:21, <Ian.Campbell@xxxxxxxxxx> wrote:
> 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.

It's the result of ditching is_64bit: Intel supports syscall only in
64-bit mode. I.e. I suppose this is an (only partially successful)
attempt to not misguide OSes that can't ever run in 64-bit mode.

Jan


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