linux-kernel - Re: [DROP][PATCH] KVM: x86/xcr0: Don't make XFEATURE_MASK

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Date:   Thu, 20 Jan 2022 20:01:05 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [DROP][PATCH] KVM: x86/xcr0: Don't make XFEATURE_MASK_SSE a
 mandatory bit setting

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;
>> }