Re: [PATCH 2/4] KVM: VMX: Refresh LBR MSR intercepts on full MSR intercept recalc

[Sean Christopherson <seanjc@google.com>]

On Tue, Oct 20, 2026, Jim Mattson wrote:
> Re-apply the LBR MSR intercepts in vmx_recalc_msr_intercepts() if the
> LBRs are currently passed through, so that changes to userspace MSR
> filters are reflected in the MSR bitmap.  Unlike SVM, which can simply
> rewrite the intercepts in either direction, leave the bitmap alone if
> the LBRs aren't passed through, as a filter can only force interception,
> never prevent it, i.e. the bitmap is already correct.  Note, clearing
> msr_passthrough to force vmx_passthrough_lbr_msrs() to re-apply the
> intercepts on the next VM-Enter isn't an option, as doing so would
> leave stale passthrough bits behind if the guest LBR event is no longer
> active.
> 
> vmx_recalc_msr_intercepts() is invoked in response to
> KVM_REQ_RECALC_INTERCEPTS, which is raised on MSR filter updates, but it
> skips the LBR MSRs, and vmx_enable_lbr_msrs_passthrough() does nothing
> if lbr_desc->msr_passthrough is already set.  If the LBR MSRs are passed
> through when userspace installs a filter that denies them, they remain
> passed through, and the guest can access them without exiting to
> userspace.  Conversely, if a filter forced interception when the guest
> LBR event became active, relaxing the filter leaves the MSRs needlessly
> intercepted.  Either way, the bitmap remains stale until the guest LBR
> event is torn down and recreated.
> 
> Only x2APIC MSRs are exempt from userspace filtering (see
> kvm_msr_allowed() and the KVM_X86_SET_MSR_FILTER documentation), and the
> filter is honored when LBR passthrough is first enabled, so KVM's
> behavior shouldn't depend on whether the filter is set before or after
> the guest enables LBRs.  Drop the LBR MSRs from the comment claiming
> that they can't be filtered by userspace, and clarify that
> lbr_desc.msr_passthrough tracks KVM's desired interception, which may
> not match the MSR bitmap when userspace filters are in play.
> 
> Fixes: 9254beaafd12 ("KVM: vmx/pmu: Reduce the overhead of LBR pass-through or cancellation")
> Cc: stable@vger.kernel.org

Meh, this isn't stable material.  Unlike SVM, where LBR virtualization is tied
to a pile of other crud, if userspace wants to filter LBRs on Intel it should
just not expose LBRs in the first place.

If it weren't for SVM, and the fact that avoiding the filters is actually more
work, I probably would vote to fix the docs.

> Signed-off-by: Jim Mattson <jmattson@google.com>
> ---
>  arch/x86/kvm/vmx/pmu_intel.c | 20 +++++++++++++++++++-
>  arch/x86/kvm/vmx/pmu_intel.h |  7 ++++++-
>  arch/x86/kvm/vmx/vmx.c       |  6 ++++--
>  arch/x86/kvm/vmx/vmx.h       |  1 +
>  4 files changed, 30 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> index 70a8c4816135..b945213f0b68 100644
> --- a/arch/x86/kvm/vmx/pmu_intel.c
> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> @@ -517,7 +517,8 @@ static void intel_pmu_refresh(struct kvm_vcpu *vcpu)
>  	memset(&lbr_desc->records, 0, sizeof(lbr_desc->records));
>  
>  	/*
> -	 * Setting passthrough of LBR MSRs is done only in the VM-Entry loop,
> +	 * LBR MSRs are initially passed through only in the VM-Entry loop (a
> +	 * full MSR intercept recalc merely refreshes an existing passthrough),
>  	 * and PMU refresh is disallowed after the vCPU has run, i.e. this code
>  	 * should never be reached while KVM is passing through MSRs.
>  	 */
> @@ -704,6 +705,23 @@ static inline void vmx_enable_lbr_msrs_passthrough(struct kvm_vcpu *vcpu)
>  	lbr_desc->msr_passthrough = true;
>  }
>  
> +/*
> + * Refresh the LBR MSR intercepts if the LBRs are passed through, so that
> + * changes to userspace MSR filters are reflected in the MSR bitmap.  There's
> + * nothing to do if the LBRs are intercepted, as a filter can only force
> + * interception, never prevent it.

Hmm, I don't love assuming that recalcs are only done for filter updates.  On the
other hand, taking action if lbr_desc->msr_passthrough=false is nonsensical.
Rather than immediately refresh the updates, what if we just disable passthrough
and let vmx_passthrough_lbr_msrs() do the actual update?

void vmx_recalc_lbr_msr_intercepts(struct kvm_vcpu *vcpu)
{
	if (WARN_ON_ONCE(!vcpu_to_lbr_desc(vcpu)))
		return;

	/* Disable passthrough to force a recalc on the next VM-Entry. */
	vmx_disable_lbr_msrs_passthrough(vcpu);
}