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

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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?

BRs,
Jing

Are 'host_xcr0' and 'xfeatures_mask_all' really interchangeable?  If so,
shouldn't we just remove 'host_xcr0' everywhere?
I think so?  But use xfeatures_mask_user().

In theory, host_xss can also be replaced with the _supervisor() and _dynamic()
variants.  That code needs a good hard look at the _dynamic() features, which is
currently just architectural LBRs.  E.g. I wouldn't be surprised if KVM currently
fails to save/restore arch LBRs due to the bit not being set in host_xss.




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