[PATCH v2] KVM: SVM: Delay restoration of host MSR_TSC_AUX until return to userspace

From: Sean Christopherson <seanjc@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH v2] KVM: SVM: Delay restoration of host MSR_TSC_AUX until return to userspace
Date: Wed, 21 Apr 2021 17:17:36 -0700	[thread overview]
Message-ID: <20210422001736.3255735-1-seanjc@google.com> (raw)

Use KVM's "user return MSRs" framework to defer restoring the host's
MSR_TSC_AUX until the CPU returns to userspace.  Add/improve comments to
clarify why MSR_TSC_AUX is intercepted on both RDMSR and WRMSR, and why
it's safe for KVM to keep the guest's value loaded even if KVM is
scheduled out.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---

v2: Rebased to kvm/queue (ish), commit 0e91d1992235 ("KVM: SVM: Allocate SEV
    command structures on local stack")
 
 arch/x86/kvm/svm/svm.c | 50 ++++++++++++++++++------------------------
 arch/x86/kvm/svm/svm.h |  7 ------
 2 files changed, 21 insertions(+), 36 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index cd8c333ed2dc..596361449f25 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -213,6 +213,15 @@ struct kvm_ldttss_desc {
 
 DEFINE_PER_CPU(struct svm_cpu_data *, svm_data);
 
+/*
+ * Only MSR_TSC_AUX is switched via the user return hook.  EFER is switched via
+ * the VMCB, and the SYSCALL/SYSENTER MSRs are handled by VMLOAD/VMSAVE.
+ *
+ * RDTSCP and RDPID are not used in the kernel, specifically to allow KVM to
+ * defer the restoration of TSC_AUX until the CPU returns to userspace.
+ */
+#define TSC_AUX_URET_SLOT	0
+
 static const u32 msrpm_ranges[] = {0, 0xc0000000, 0xc0010000};
 
 #define NUM_MSR_MAPS ARRAY_SIZE(msrpm_ranges)
@@ -958,6 +967,9 @@ static __init int svm_hardware_setup(void)
 		kvm_tsc_scaling_ratio_frac_bits = 32;
 	}
 
+	if (boot_cpu_has(X86_FEATURE_RDTSCP))
+		kvm_define_user_return_msr(TSC_AUX_URET_SLOT, MSR_TSC_AUX);
+
 	/* Check for pause filtering support */
 	if (!boot_cpu_has(X86_FEATURE_PAUSEFILTER)) {
 		pause_filter_count = 0;
@@ -1423,19 +1435,10 @@ static void svm_prepare_guest_switch(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
 	struct svm_cpu_data *sd = per_cpu(svm_data, vcpu->cpu);
-	unsigned int i;
 
 	if (svm->guest_state_loaded)
 		return;
 
-	/*
-	 * Certain MSRs are restored on VMEXIT (sev-es), or vmload of host save
-	 * area (non-sev-es). Save ones that aren't so we can restore them
-	 * individually later.
-	 */
-	for (i = 0; i < NR_HOST_SAVE_USER_MSRS; i++)
-		rdmsrl(host_save_user_msrs[i], svm->host_user_msrs[i]);
-
 	/*
 	 * Save additional host state that will be restored on VMEXIT (sev-es)
 	 * or subsequent vmload of host save area.
@@ -1454,29 +1457,15 @@ static void svm_prepare_guest_switch(struct kvm_vcpu *vcpu)
 		}
 	}
 
-	/* This assumes that the kernel never uses MSR_TSC_AUX */
 	if (static_cpu_has(X86_FEATURE_RDTSCP))
-		wrmsrl(MSR_TSC_AUX, svm->tsc_aux);
+		kvm_set_user_return_msr(TSC_AUX_URET_SLOT, svm->tsc_aux, -1ull);
 
 	svm->guest_state_loaded = true;
 }
 
 static void svm_prepare_host_switch(struct kvm_vcpu *vcpu)
 {
-	struct vcpu_svm *svm = to_svm(vcpu);
-	unsigned int i;
-
-	if (!svm->guest_state_loaded)
-		return;
-
-	/*
-	 * Certain MSRs are restored on VMEXIT (sev-es), or vmload of host save
-	 * area (non-sev-es). Restore the ones that weren't.
-	 */
-	for (i = 0; i < NR_HOST_SAVE_USER_MSRS; i++)
-		wrmsrl(host_save_user_msrs[i], svm->host_user_msrs[i]);
-
-	svm->guest_state_loaded = false;
+	to_svm(vcpu)->guest_state_loaded = false;
 }
 
 static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
@@ -2893,12 +2882,15 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
 			return 1;
 
 		/*
-		 * This is rare, so we update the MSR here instead of using
-		 * direct_access_msrs.  Doing that would require a rdmsr in
-		 * svm_vcpu_put.
+		 * TSC_AUX is usually changed only during boot and never read
+		 * directly.  Intercept TSC_AUX instead of exposing it to the
+		 * guest via direct_acess_msrs, and switch it via user return.
 		 */
 		svm->tsc_aux = data;
-		wrmsrl(MSR_TSC_AUX, svm->tsc_aux);
+
+		preempt_disable();
+		kvm_set_user_return_msr(TSC_AUX_URET_SLOT, data, -1ull);
+		preempt_enable();
 		break;
 	case MSR_IA32_DEBUGCTLMSR:
 		if (!boot_cpu_has(X86_FEATURE_LBRV)) {
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 454da1c1d9b7..9dce6f290041 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -23,11 +23,6 @@
 
 #define __sme_page_pa(x) __sme_set(page_to_pfn(x) << PAGE_SHIFT)
 
-static const u32 host_save_user_msrs[] = {
-	MSR_TSC_AUX,
-};
-#define NR_HOST_SAVE_USER_MSRS ARRAY_SIZE(host_save_user_msrs)
-
 #define	IOPM_SIZE PAGE_SIZE * 3
 #define	MSRPM_SIZE PAGE_SIZE * 2
 
@@ -128,8 +123,6 @@ struct vcpu_svm {
 
 	u64 next_rip;
 
-	u64 host_user_msrs[NR_HOST_SAVE_USER_MSRS];
-
 	u64 spec_ctrl;
 	/*
 	 * Contains guest-controlled bits of VIRT_SPEC_CTRL, which will be
-- 
2.31.1.498.g6c1eba8ee3d-goog

