[PATCH 6/8] kvm: svm: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD down to guest

[Paolo Bonzini]

Direct access to MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD is important
for performance.  Allow load/store of MSR_IA32_SPEC_CTRL, restore guest
IBRS on VM entry and set it to 0 on VM exit (because Linux does not use
it yet).

Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 arch/x86/kvm/svm.c | 42 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 31ace8d7774a..934a21e02e03 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -183,6 +183,8 @@ struct vcpu_svm {
 		u64 gs_base;
 	} host;
 
+	u64 spec_ctrl;
+
 	u32 *msrpm;
 
 	ulong nmi_iret_rip;
@@ -248,6 +250,8 @@ struct amd_svm_iommu_ir {
 	{ .index = MSR_CSTAR,				.always = true  },
 	{ .index = MSR_SYSCALL_MASK,			.always = true  },
 #endif
+	{ .index = MSR_IA32_SPEC_CTRL,			.always = true  },
+	{ .index = MSR_IA32_PRED_CMD,			.always = true  },
 	{ .index = MSR_IA32_LASTBRANCHFROMIP,		.always = false },
 	{ .index = MSR_IA32_LASTBRANCHTOIP,		.always = false },
 	{ .index = MSR_IA32_LASTINTFROMIP,		.always = false },
@@ -283,6 +287,8 @@ struct amd_svm_iommu_ir {
 /* enable/disable Virtual GIF */
 static int vgif = true;
 module_param(vgif, int, 0444);
+ 
+static bool __read_mostly have_spec_ctrl;
 
 static void svm_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0);
 static void svm_flush_tlb(struct kvm_vcpu *vcpu, bool invalidate_gpa);
@@ -1135,6 +1141,17 @@ static __init int svm_hardware_setup(void)
 			pr_info("Virtual GIF supported\n");
 	}
 
+	/*
+	 * FIXME: this is only needed until SPEC_CTRL is supported
+	 * by upstream Linux in cpufeatures, then it can be replaced
+	 * with static_cpu_has.
+	 */
+	have_spec_ctrl = cpu_has_spec_ctrl();
+	if (have_spec_ctrl)
+		pr_info("kvm: SPEC_CTRL available\n");
+	else
+		pr_info("kvm: SPEC_CTRL not available\n");
+
 	return 0;
 
 err:
@@ -3599,6 +3616,9 @@ static int svm_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 	case MSR_VM_CR:
 		msr_info->data = svm->nested.vm_cr_msr;
 		break;
+	case MSR_IA32_SPEC_CTRL:
+		msr_info->data = svm->spec_ctrl;
+		break;
 	case MSR_IA32_UCODE_REV:
 		msr_info->data = 0x01000065;
 		break;
@@ -3754,6 +3774,9 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
 	case MSR_VM_IGNNE:
 		vcpu_unimpl(vcpu, "unimplemented wrmsr: 0x%x data 0x%llx\n", ecx, data);
 		break;
+	case MSR_IA32_SPEC_CTRL:
+		svm->spec_ctrl = data;
+		break;
 	case MSR_IA32_APICBASE:
 		if (kvm_vcpu_apicv_active(vcpu))
 			avic_update_vapic_bar(to_svm(vcpu), data);
@@ -4942,6 +4965,13 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
 
 	local_irq_enable();
 
+	/*
+	 * MSR_IA32_SPEC_CTRL is restored after the last indirect branch
+	 * before vmentry.
+	 */
+	if (have_spec_ctrl && svm->spec_ctrl != 0)
+		wrmsrl(MSR_IA32_SPEC_CTRL, svm->spec_ctrl);
+
 	asm volatile (
 		"push %%" _ASM_BP "; \n\t"
 		"mov %c[rbx](%[svm]), %%" _ASM_BX " \n\t"
@@ -5015,6 +5045,18 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
 #endif
 		);
 
+	if (have_spec_ctrl) {
+		rdmsrl(MSR_IA32_SPEC_CTRL, svm->spec_ctrl);
+		if (svm->spec_ctrl != 0)
+			wrmsrl(MSR_IA32_SPEC_CTRL, 0);
+	}
+	/*
+	 * Speculative execution past the above wrmsrl might encounter
+	 * an indirect branch and use guest-controlled contents of the
+	 * indirect branch predictor; block it.
+	 */
+	asm("lfence");
+
 #ifdef CONFIG_X86_64
 	wrmsrl(MSR_GS_BASE, svm->host.gs_base);
 #else
-- 
1.8.3.1