[PATCH v2] kvm: svm: Only propagate next_rip when guest supports it

[Joerg Roedel <joro@8bytes.org>]

On Fri, Oct 09, 2015 at 01:15:11PM +0200, Paolo Bonzini wrote:
> This could be a bit expensive to do on every vmexit.  Can you benchmark
> it with kvm-unit-tests, or just cache the result in struct vcpu_svm?

Yes, caching it is certainly a good idea. I updated the patch:

>From 94ee662c527683c26ea5fa98a5a8f2c798c58470 Mon Sep 17 00:00:00 2001
From: Joerg Roedel <jroedel@suse.de>
Date: Wed, 7 Oct 2015 13:38:19 +0200
Subject: [PATCH] kvm: svm: Only propagate next_rip when guest supports it

Currently we always write the next_rip of the shadow vmcb to
the guests vmcb when we emulate a vmexit. This could confuse
the guest when its cpuid indicated no support for the
next_rip feature.

Fix this by only propagating next_rip if the guest actually
supports it.

Cc: Bandan Das <bsd@redhat.com>
Cc: Dirk Mueller <dmueller@suse.com>
Tested-By: Dirk Mueller <dmueller@suse.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
 arch/x86/kvm/cpuid.h | 21 +++++++++++++++++++++
 arch/x86/kvm/svm.c   | 11 ++++++++++-
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
index dd05b9c..effca1f 100644
--- a/arch/x86/kvm/cpuid.h
+++ b/arch/x86/kvm/cpuid.h
@@ -133,4 +133,25 @@ static inline bool guest_cpuid_has_mpx(struct kvm_vcpu *vcpu)
 	best = kvm_find_cpuid_entry(vcpu, 7, 0);
 	return best && (best->ebx & bit(X86_FEATURE_MPX));
 }
+
+/*
+ * NRIPS is provided through cpuidfn 0x8000000a.edx bit 3
+ */
+#define BIT_NRIPS	3
+
+static inline bool guest_cpuid_has_nrips(struct kvm_vcpu *vcpu)
+{
+	struct kvm_cpuid_entry2 *best;
+
+	best = kvm_find_cpuid_entry(vcpu, 0x8000000a, 0);
+
+	/*
+	 * NRIPS is a scattered cpuid feature, so we can't use
+	 * X86_FEATURE_NRIPS here (X86_FEATURE_NRIPS would be bit
+	 * position 8, not 3).
+	 */
+	return best && (best->edx & bit(BIT_NRIPS));
+}
+#undef BIT_NRIPS
+
 #endif
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 2f9ed1f..e9e3294 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -159,6 +159,9 @@ struct vcpu_svm {
 	u32 apf_reason;
 
 	u64  tsc_ratio;
+
+	/* cached guest cpuid flags for faster access */
+	bool nrips_enabled	: 1;
 };
 
 static DEFINE_PER_CPU(u64, current_tsc_ratio);
@@ -2365,7 +2368,9 @@ static int nested_svm_vmexit(struct vcpu_svm *svm)
 	nested_vmcb->control.exit_info_2       = vmcb->control.exit_info_2;
 	nested_vmcb->control.exit_int_info     = vmcb->control.exit_int_info;
 	nested_vmcb->control.exit_int_info_err = vmcb->control.exit_int_info_err;
-	nested_vmcb->control.next_rip          = vmcb->control.next_rip;
+
+	if (svm->nrips_enabled)
+		nested_vmcb->control.next_rip  = vmcb->control.next_rip;
 
 	/*
 	 * If we emulate a VMRUN/#VMEXIT in the same host #vmexit cycle we have
@@ -4098,6 +4103,10 @@ static u64 svm_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
 
 static void svm_cpuid_update(struct kvm_vcpu *vcpu)
 {
+	struct vcpu_svm *svm = to_svm(vcpu);
+
+	/* Update nrips enabled cache */
+	svm->nrips_enabled = !!guest_cpuid_has_nrips(&svm->vcpu);
 }
 
 static void svm_set_supported_cpuid(u32 func, struct kvm_cpuid_entry2 *entry)
-- 
1.8.4.5