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    SubjectRe: [PATCH v1] kvm: x86: Revise guest_fpu xcomp_bv field
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    On 2/23/2021 12:06 AM, Sean Christopherson wrote:
    > On Mon, Feb 22, 2021, Liu, Jing2 wrote:
    >> On 2/9/2021 1:24 AM, Sean Christopherson wrote:
    >>> On Mon, Feb 08, 2021, Dave Hansen wrote:
    >>>> On 2/8/21 8:16 AM, Jing Liu wrote:
    >>>>> -#define XSTATE_COMPACTION_ENABLED (1ULL << 63)
    >>>>> -
    >>>>> static void fill_xsave(u8 *dest, struct kvm_vcpu *vcpu)
    >>>>> {
    >>>>> struct xregs_state *xsave = &vcpu->arch.guest_fpu->state.xsave;
    >>>>> @@ -4494,7 +4492,8 @@ static void load_xsave(struct kvm_vcpu *vcpu, u8 *src)
    >>>>> /* Set XSTATE_BV and possibly XCOMP_BV. */
    >>>>> xsave->header.xfeatures = xstate_bv;
    >>>>> if (boot_cpu_has(X86_FEATURE_XSAVES))
    >>>>> - xsave->header.xcomp_bv = host_xcr0 | XSTATE_COMPACTION_ENABLED;
    >>>>> + xsave->header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT |
    >>>>> + xfeatures_mask_all;
    >>> This is wrong, xfeatures_mask_all also tracks supervisor states.
    >> When looking at SDM Vol2 XSAVES instruction Operation part, it says as
    >> follows,
    >>
    >> RFBM ← (XCR0 OR IA32_XSS) AND EDX:EAX;
    >> COMPMASK ← RFBM OR 80000000_00000000H;
    >> ...
    >>
    >> XCOMP_BV field in XSAVE header ← COMPMASK;
    >>
    >>
    >> So it seems xcomp_bv also tracks supervisor states?
    > Yes, sorry, I got distracted by Dave's question and didn't read the changelog
    > closely.
    >
    > Now that I have, I find "Since fpstate_init() has initialized xcomp_bv, let's
    > just use that." confusing. I think what you intend to say is that we can use
    > the same _logic_ as fpstate_init_xstate() for calculating xcomp_bv.
    Yes, that's the idea.
    >
    > That said, it would be helpful for the changelog to explain why it's correct to
    > use xfeatures_mask_all, e.g. just a short comment stating that the variable holds
    > all XCR0 and XSS bits enabled by the host kernel. Justifying a change with
    > "because other code does it" is sketchy, becuse there's no guarantee that what
    > something else does is also correct for KVM, or that the existing code itself is
    > even correct.
    Got it, thanks for the details on this.
    Then how about making the commit message like,

    XCOMP_BV[63] field indicates that the save area is in the
    compacted format and XCOMP_BV[62:0] indicates the states that
    have space allocated in the save area, including both XCR0
    and XSS bits enable by the host kernel. Use xfeatures_mask_all
    for calculating xcomp_bv and reuse XCOMP_BV_COMPACTED_FORMAT
    defined by kernel.

    Thanks,
    Jing


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