Re: [PATCH v7 1/7] KVM: CPUID: Fix IA32_XSS support in CPUID(0xd,i) enumeration

[Yang Weijiang]

On Tue, Oct 22, 2019 at 12:46:15PM -0700, Sean Christopherson wrote:
> On Fri, Oct 18, 2019 at 09:28:09AM +0800, Yang Weijiang wrote:
> > On Thu, Oct 17, 2019 at 12:46:22PM -0700, Sean Christopherson wrote:
> > > On Wed, Oct 02, 2019 at 10:26:10AM -0700, Jim Mattson wrote:
> > > > > +                       entry->eax = 0;
> > > > > +                       entry->ebx = 0;
> > > > > +                       entry->ecx = 0;
> > > > > +                       entry->edx = 0;
> > > > > +                       return;
> > > > > +               }
> > > > > +               if (entry->ecx)
> > > > > +                       entry->ebx = 0;
> > > > 
> > > > This seems to back up my claims above regarding the EBX output for
> > > > cases 0 and 1, but aside from those subleaves, is this correct? For
> > > > subleaves > 1, ECX bit 1 can be set for extended state components that
> > > > need to be cache-line aligned. Such components could map to a valid
> > > > bit in XCR0 and have a non-zero offset from the beginning of the
> > > > non-compacted XSAVE area.
> > > > 
> > > > > +               entry->edx = 0;
> > > > 
> > > > This seems too aggressive. See my comments above regarding EDX outputs
> > > > for cases 0 and 1.
> > > > 
> > Sean, I don't know how to deal with entry->edx here as SDM says it's
> > reserved for valid subleaf.
> 
> The SDM also states:
> 
>   Bit 31 - 00: Reports the supported bits of the upper 32 bits of XCR0.
>   XCR0[n+32] can be set to 1 only if EDX[n] is 1.
> 
> the second part, "Bits 31 - 00: Reserved" is at best superfluous, e.g. it
> could be interpreted as saying that XCR0[63:32] are currently reserved,
> and at worst the extra qualifier is an SDM bug and should be removed.
> 
> TL;DR: Ignore the blurb about the bits being reserved.
Thanks Sean, I'll follow it.