Re: [PATCH v4 4/4] KVM: SVM: Add Bus Lock Detect support

[Sean Christopherson]

On Tue, Nov 11, 2025, Shivansh Dhiman wrote:
> On 17-08-2024 05:43, Sean Christopherson wrote:
> > On Thu, Aug 08, 2024, Ravi Bangoria wrote:
> >> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> >> index e1b6a16e97c0..9f3d31a5d231 100644
> >> --- a/arch/x86/kvm/svm/svm.c
> >> +++ b/arch/x86/kvm/svm/svm.c
> >> @@ -1047,7 +1047,8 @@ void svm_update_lbrv(struct kvm_vcpu *vcpu)
> >>  {
> >>  	struct vcpu_svm *svm = to_svm(vcpu);
> >>  	bool current_enable_lbrv = svm->vmcb->control.virt_ext & LBR_CTL_ENABLE_MASK;
> >> -	bool enable_lbrv = (svm_get_lbr_vmcb(svm)->save.dbgctl & DEBUGCTLMSR_LBR) ||
> >> +	u64 dbgctl_buslock_lbr = DEBUGCTLMSR_BUS_LOCK_DETECT | DEBUGCTLMSR_LBR;
> >> +	bool enable_lbrv = (svm_get_lbr_vmcb(svm)->save.dbgctl & dbgctl_buslock_lbr) ||
> >>  			    (is_guest_mode(vcpu) && guest_can_use(vcpu, X86_FEATURE_LBRV) &&
> >>  			    (svm->nested.ctl.virt_ext & LBR_CTL_ENABLE_MASK));
> > 
> > Out of sight, but this leads to calling svm_enable_lbrv() even when the guest
> > just wants to enable BUS_LOCK_DETECT.  Ignoring SEV-ES guests, KVM will intercept
> > writes to DEBUGCTL, so can't KVM defer mucking with the intercepts and
> > svm_copy_lbrs() until the guest actually wants to use LBRs?
> > 
> > Hmm, and I think the existing code is broken.  If L1 passes DEBUGCTL through to
> > L2, then KVM will handles writes to L1's effective value.  And if L1 also passes
> > through the LBRs, then KVM will fail to update the MSR bitmaps for vmcb02.
> > 
> > Ah, it's just a performance issue though, because KVM will still emulate RDMSR.
> > 
> > Ugh, this code is silly.  The LBR MSRs are read-only, yet KVM passes them through
> > for write.
> > 
> > Anyways, I'm thinking something like this?  Note, using msr_write_intercepted()
> > is wrong, because that'll check L2's bitmap if is_guest_mode(), and the idea is
> > to use L1's bitmap as the canary.

...

> ===========================================================
> Issue 1: Interception still enabled after enabling LBRV
> ===========================================================
> Using the 6.16 upstream kernel (unpatched) I ran the KUT tests and they passed
> when run from both the bare metal and from inside a L1 guest. However for L2
> guest, when looking at the logs I found that RDMSR interception of LBR MSRs is
> still enabled despite the LBRV is enabled for the L2 guest. Effectively, the
> reads are emulated instead of being virtualized, which is not the intended
> behaviour. KUT cannot distinguish between emulated and virtualized RDMSR, and
> hence the test passes regardless.

I haven't looked closely at your patch or at Yosry's patches, but I suspect this
was _just_ fixed:

https://lore.kernel.org/all/20251108004524.1600006-1-yosry.ahmed@xxxxxxxxx

> ===========================================================
> Issue 2: Basic LBR KUT fails with Sean's implementation
> ===========================================================
> After using your implementation, all KUTs passed on the bare metal. With LBRV
> enabled for L2, RDMSR interception of LBR MSRs is disabled as intended.
> However, when running KUT tests inside an L1 guest, the tests fail.

Same story here: I haven't had cycles to actually look at code, but Yosry also
posted a pile of changes for KUT:

https://lore.kernel.org/all/20251110232642.633672-1-yosry.ahmed@xxxxxxxxx