Re: [PATCH v1] kvm: x86: Revise guest_fpu xcomp_bv field

[Liu, Jing2]

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