Messages in this thread | | | Subject | Re: [PATCH v1] kvm: x86: Revise guest_fpu xcomp_bv field | From | "Liu, Jing2" <> | Date | Tue, 23 Feb 2021 11:06:14 +0800 |
| |
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
| |