Re: [DROP][PATCH] KVM: x86/xcr0: Don't make XFEATURE_MASK_SSE a mandatory bit setting

[Like Xu]

On 19/1/2022 11:23 pm, Dave Hansen wrote:
> On 1/18/22 11:04 PM, Like Xu wrote:
> > Remove the XFEATURE_MASK_SSE bit as part of the XFEATURE_MASK_EXTEND
> > and opportunistically, move it into the context of its unique user KVM.
>
> Is this a problem for xstate_required_size()?  The rules for the CPUID
> sub-functions <=1 are different than those for >1.  Most importantly,
> 'eax' doesn't enumerate the size of the feature for the XFEATURE_SSE
> sub-leaf.

Indeed.

> I think XFEATURE_MASK_EXTEND was being used to avoid that oddity:

It seems that the cpuid.0xd.0.ebx size update for the SSE+AVX state needs
to be triggered by setting bit 2 which is quite odd:

 XCR0 = 001B, ebx=00000240
 XCR0 = 011B, ebx=00000240
 XCR0 = 111B, ebx=00000340

Thank you and sorry for the noise.

> > u32 xstate_required_size(u64 xstate_bv, bool compacted)
> > {
> >          int feature_bit = 0;
> >          u32 ret = XSAVE_HDR_SIZE + XSAVE_HDR_OFFSET;
> >
> >          xstate_bv &= XFEATURE_MASK_EXTEND;
> >          while (xstate_bv) {
> >                  if (xstate_bv & 0x1) {
> >                          u32 eax, ebx, ecx, edx, offset;
> >                          cpuid_count(0xD, feature_bit, &eax, &ebx, &ecx, &edx);
> >                          /* ECX[1]: 64B alignment in compacted form */
> >                          if (compacted)
> >                                  offset = (ecx & 0x2) ? ALIGN(ret, 64) : ret;
> >                          else
> >                                  offset = ebx;
> >                          ret = max(ret, offset + eax);
> >                  }
> >
> >                  xstate_bv >>= 1;
> >                  feature_bit++;
> >          }
> >
> >          return ret;
> > }