From: Sean Christopherson <seanjc@google.com>
To: sashiko-reviews@lists.linux.dev
Cc: kvm@vger.kernel.org
Subject: Re: [PATCH v6 12/12] KVM: SVM: Add support for virtualizating Bus Lock Detect
Date: Wed, 30 Sep 2026 13:20:23 -0700 [thread overview]
Message-ID: <ar1vB2BtBggBIxzp@google.com> (raw)
In-Reply-To: <20260930195407.28A811F000FF@smtp.kernel.org>
On Wed, Sep 30, 2026, sashiko-bot@kernel.org wrote:
> > diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> > index c3380d865b751..6d0242966dc87 100644
> > --- a/arch/x86/kvm/svm/svm.h
> > +++ b/arch/x86/kvm/svm/svm.h
> [ ... ]
> > @@ -789,7 +789,12 @@ BUILD_SVM_MSR_BITMAP_HELPERS(void, set, __set)
> >
> > static inline u64 svm_get_supported_debugctl(struct kvm_vcpu *vcpu)
> > {
> > - return DEBUGCTLMSR_LBR;
> > + u64 debugctl = DEBUGCTLMSR_LBR;
> > +
> > + if (guest_cpu_cap_has(vcpu, X86_FEATURE_BUS_LOCK_DETECT))
> > + debugctl |= DEBUGCTLMSR_BUS_LOCK_DETECT;
> > +
>
> [Severity: Medium]
> Does this restrict host-initiated MSR restoration? By making the
> DEBUGCTLMSR_BUS_LOCK_DETECT bit strictly dependent on guest_cpu_cap_has(),
> this might reject host-initiated writes to MSR_IA32_DEBUGCTLMSR if the VMM
> restores MSRs before initializing the guest CPUID.
>
> In svm_set_msr(), the rejection happens unconditionally:
>
> svm_set_msr()
> if (data & ~svm_get_supported_debugctl(vcpu))
> return 1;
>
> Since svm_set_msr() does not appear to check msr->host_initiated before
> enforcing this capability limit, could this regression break live
> migration when userspace sends KVM_SET_MSRS before the CPUID is fully
> configured?
By design. The msr->host_initiated exemption is "legacy" KVM behavior that we're
actively trying to remove.
next prev parent reply other threads:[~2026-09-30 20:20 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-09-30 19:36 [PATCH v6 00/12] KVM: x86: Bus Lock Detect fixes and SVM support Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 01/12] KVM: x86: Preserve DR6.BLD (Bus Lock Detect) when delivering #DB payload Sean Christopherson
2026-09-30 23:46 ` Shivansh Dhiman
2026-09-30 19:36 ` [PATCH v6 02/12] KVM: x86: Rename kvm_dr6_fixed() => kvm_get_dr6_fixed_1() Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 03/12] KVM: Return an "unsigned long", not "u64" for the fixed-1 DR6 bits Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 04/12] KVM: x86: Force fixed-1 bits in DR6 after synchronizing with hardware Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 05/12] KVM: x86: Set guest's fixed-1 DR6 bits when delivering #DB payload Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 06/12] KVM: x86: Kill off DR6_FIXED_1 to prevent future misuse Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 07/12] KVM: SVM: Add helper to query if LBR virtualization needs to be enabled Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 08/12] KVM: nSVM: Disable LBRV in nested control cache when unsupported Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 09/12] KVM: nSVM: Open code check on LBR virtualization being enabled in vmcb12 Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 10/12] KVM: nSVM: Don't assume all active-low bits DR6 are fixed-1 Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 11/12] KVM: SVM: Add a vCPU-aware helper to get supported DEBUGCTL bits Sean Christopherson
2026-09-30 19:36 ` [PATCH v6 12/12] KVM: SVM: Add support for virtualizating Bus Lock Detect Sean Christopherson
2026-09-30 19:54 ` sashiko-bot
2026-09-30 20:20 ` Sean Christopherson [this message]
2026-10-02 21:07 ` [PATCH v6 00/12] KVM: x86: Bus Lock Detect fixes and SVM support Sean Christopherson
2026-10-05 5:46 ` Shivansh Dhiman
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