Re: [Xen-devel] [PATCH V4 6/6] x86/hvm: pkeys, add pkeys support for cpuid handling

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.12.15 at 10:25, <huaitong.han@xxxxxxxxx> wrote:
> On Tue, 2015-12-22 at 01:30 -0700, Jan Beulich wrote:
>> > > > On 22.12.15 at 03:54, <huaitong.han@xxxxxxxxx> wrote:
>> > On Mon, 2015-12-21 at 08:07 -0700, Jan Beulich wrote:
>> > > > > > On 21.12.15 at 08:21, <huaitong.han@xxxxxxxxx> wrote:
>> > > > @@ -4600,6 +4600,14 @@ void hvm_cpuid(unsigned int input,
>> > > > unsigned
>> > > > int *eax, unsigned int *ebx,
>> > > >          /* Don't expose INVPCID to non-hap hvm. */
>> > > >          if ( (count == 0) && !hap_enabled(d) )
>> > > >              *ebx &= ~cpufeat_mask(X86_FEATURE_INVPCID);
>> > > > +
>> > > > +        /* X86_FEATURE_PKU is not yet implemented for shadow
>> > > > paging. */
>> > > > +        if ( (count == 0) && !hap_enabled(d) )
>> > > > +            *ecx &= ~cpufeat_mask(X86_FEATURE_PKU);
>> > > 
>> > > I'm still missing the xsave dependency here.
>> > Xsave dependency deletion becasue we use RDPKRU to get PKRU
>> > register
>> > value instead of XSAVE now.
>> 
>> What the hypervisor does doesn't matter here. The question is
>> whether from an architectural standpoint XSAVE is a prerequsite.
>> If it is, then you need to clear PKU when _guest_ XSAVE is clear.
> No, XSAVE is not a prerequsite.

I.e. OSes are expected to deal with both cases (PKU w/ XSAVE and
PKU w/o XSAVE)? Sounds like a recipe for trouble, but well - okay.

Jan


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