[PATCH 1/4] KVM: SVM: Always update LBR MSR intercepts on full MSR intercept recalc

[Jim Mattson <jmattson@google.com>]

Split the "already up-to-date" check out of the code that writes the LBR
MSR intercepts, and write the intercepts unconditionally from
svm_recalc_msr_intercepts(), so that changes to userspace MSR filters are
reflected in the MSR permissions map.

svm_recalc_msr_intercepts() is invoked in response to
KVM_REQ_RECALC_INTERCEPTS, which is raised on MSR filter updates.  But
svm->lbr_msrs_intercepted tracks only the interception KVM wants, not
what is actually in the bitmap after the filter is applied.  If the LBR
MSRs are passed through when userspace installs a filter that denies
them, the early return leaves them passed through, and the guest can
access them without exiting to userspace.  Conversely, if a filter
forced interception when LBR virtualization was enabled, relaxing the
filter leaves the MSRs needlessly intercepted.  Either way, the bitmap
remains stale until the next change to LBR_CTL.

Full intercept recalculations are rare (e.g. on RESET/INIT, EFER.SVME
toggles, and CPUID, PERF_CAPABILITIES, and MSR filter updates), and
never happen on nested VMRUN or #VMEXIT, so writing the intercepts there
does not reintroduce the redundant per-VMRUN bitmap updates that the
check was added to avoid.  And because svm_recalc_msr_intercepts()
already unconditionally writes the intercepts for many other MSRs (and
thus marks Hyper-V's nested enlightenments dirty), writing the LBR MSR
intercepts there doesn't introduce any new invalidations.

Add a comment to clarify that lbr_msrs_intercepted reflects KVM's
desired interception, not necessarily the state of the bitmap.

Fixes: 3fa05f96fc08 ("KVM: SVM: Fix redundant updates of LBR MSR intercepts")
Cc: stable@vger.kernel.org
Signed-off-by: Jim Mattson <jmattson@google.com>
---
 arch/x86/kvm/svm/svm.c | 28 +++++++++++++++++++++++-----
 arch/x86/kvm/svm/svm.h |  6 ++++++
 2 files changed, 29 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index fe0cba4731b2..55b3ee840365 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -721,14 +721,11 @@ void *svm_alloc_permissions_map(unsigned long size, gfp_t gfp_mask)
 	return pm;
 }
 
-static void svm_recalc_lbr_msr_intercepts(struct kvm_vcpu *vcpu)
+static void svm_set_lbr_msr_intercepts(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
 	bool intercept = !(svm->vmcb->control.misc_ctl2 & SVM_MISC2_ENABLE_V_LBR);
 
-	if (intercept == svm->lbr_msrs_intercepted)
-		return;
-
 	svm_set_intercept_for_msr(vcpu, MSR_IA32_LASTBRANCHFROMIP, MSR_TYPE_RW, intercept);
 	svm_set_intercept_for_msr(vcpu, MSR_IA32_LASTBRANCHTOIP, MSR_TYPE_RW, intercept);
 	svm_set_intercept_for_msr(vcpu, MSR_IA32_LASTINTFROMIP, MSR_TYPE_RW, intercept);
@@ -740,6 +737,22 @@ static void svm_recalc_lbr_msr_intercepts(struct kvm_vcpu *vcpu)
 	svm->lbr_msrs_intercepted = intercept;
 }
 
+static void svm_recalc_lbr_msr_intercepts(struct kvm_vcpu *vcpu)
+{
+	struct vcpu_svm *svm = to_svm(vcpu);
+	bool intercept = !(svm->vmcb->control.misc_ctl2 & SVM_MISC2_ENABLE_V_LBR);
+
+	/*
+	 * Skip redundant updates from svm_update_lbrv(), e.g. on every nested
+	 * VMRUN and #VMEXIT.  Note, svm_recalc_msr_intercepts() updates the
+	 * bitmap unconditionally, as userspace MSR filters may have changed.
+	 */
+	if (intercept == svm->lbr_msrs_intercepted)
+		return;
+
+	svm_set_lbr_msr_intercepts(vcpu);
+}
+
 void svm_vcpu_free_msrpm(void *msrpm)
 {
 	__free_pages(virt_to_page(msrpm), get_order(MSRPM_SIZE));
@@ -795,8 +808,13 @@ static void svm_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
 	svm_disable_intercept_for_msr(vcpu, MSR_SYSCALL_MASK, MSR_TYPE_RW);
 #endif
 
+	/*
+	 * Unconditionally refresh the LBR MSR intercepts, as userspace MSR
+	 * filters may have changed, in which case the bitmap may be stale even
+	 * if KVM's desired interception hasn't changed.
+	 */
 	if (lbrv)
-		svm_recalc_lbr_msr_intercepts(vcpu);
+		svm_set_lbr_msr_intercepts(vcpu);
 
 	if (cpu_feature_enabled(X86_FEATURE_IBPB))
 		svm_set_intercept_for_msr(vcpu, MSR_IA32_PRED_CMD, MSR_TYPE_W,
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 84f19026d3e8..62e08d6d4706 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -364,6 +364,12 @@ struct vcpu_svm {
 
 	bool avic_irq_window;
 	bool x2avic_msrs_intercepted;
+
+	/*
+	 * True if KVM wants the LBR MSRs to be intercepted.  Note, userspace MSR
+	 * filters may force interception regardless, i.e. this doesn't
+	 * necessarily reflect the state of the MSR bitmap.
+	 */
 	bool lbr_msrs_intercepted;
 
 	/* Guest GIF value, used when vGIF is not enabled */
-- 
2.56.0.212.g129ede20d9-goog