linux-kernel - Re: [PATCH v3 07/21] KVM:x86: Refresh CPUID on write to guest MSR_IA32

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Message-ID: <ZIuitcH7P1JpkzFH@google.com>
Date:   Thu, 15 Jun 2023 16:45:57 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Weijiang Yang <weijiang.yang@...el.com>
Cc:     Chao Gao <chao.gao@...el.com>, pbonzini@...hat.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        peterz@...radead.org, rppt@...nel.org, binbin.wu@...ux.intel.com,
        rick.p.edgecombe@...el.com, john.allen@....com,
        Zhang Yi Z <yi.z.zhang@...ux.intel.com>
Subject: Re: [PATCH v3 07/21] KVM:x86: Refresh CPUID on write to guest MSR_IA32_XSS

On Wed, May 31, 2023, Weijiang Yang wrote:
> 
> On 5/30/2023 8:08 PM, Chao Gao wrote:
> > > > > --- a/arch/x86/kvm/x86.c
> > > > > +++ b/arch/x86/kvm/x86.c
> > > > > @@ -3776,8 +3776,10 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> > > > > 		 */
> > > > > 		if (data & ~kvm_caps.supported_xss)
> > > > Shouldn't we check against the supported value of _this_ guest? similar to
> > > > guest_supported_xcr0.
> > > I don't think it requires an extra variable to serve per guest purpose.
> > > 
> > > For guest XSS settings, now we don't add extra constraints like XCR0, thus
> > QEMU can impose constraints by configuring guest CPUID.0xd.1 to indicate
> > certain supervisor state components cannot be managed by XSAVES, even
> > though KVM supports them. IOW, guests may differ in the supported values
> > for the IA32_XSS MSR.
> 
> OK, will change this part to align with xcr0 settings. Thanks!

Please write KVM-Unit-Tests to verify KVM correctly handles the various MSRs related
to CET, e.g. a test_cet_msrs() subtest in msr.c would do nicely.  Hmm, though testing
the combinations of CPUID bits will require multiple x86/unittests.cfg entries.
Might be time to split up msr.c into a library and then multiple tests.