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@xxxxxxx>
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@xxxxxxxxxx>
Cc: Dirk Mueller <dmueller@xxxxxxxx>
Tested-By: Dirk Mueller <dmueller@xxxxxxxx>
Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
---
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
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