linux-kernel - [PATCH v2] KVM: x86: do not read FS/GS base MSRs when saving them

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Date:   Tue,  2 Jan 2018 14:59:51 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     luto@...nel.org
Subject: [PATCH v2] KVM: x86: do not read FS/GS base MSRs when saving them

The FS and userspace GS bases are available in current->thread, while the
kernel GS base is a percpu variable.  Skip the expensive rdmsr and just
get the values from memory.

Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
	v1->v2: hide the accessor for 32-bit kernels

 arch/x86/include/asm/desc.h     |  8 ++++++++
 arch/x86/include/asm/kvm_host.h | 10 ----------
 arch/x86/kernel/cpu/common.c    |  1 +
 arch/x86/kvm/svm.c              |  2 +-
 arch/x86/kvm/vmx.c              | 17 +++--------------
 5 files changed, 13 insertions(+), 25 deletions(-)

diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
index 4011cb03ef08..6ef8c47d0baa 100644
--- a/arch/x86/include/asm/desc.h
+++ b/arch/x86/include/asm/desc.h
@@ -85,6 +85,14 @@ static inline phys_addr_t get_cpu_gdt_paddr(unsigned int cpu)
 	return per_cpu_ptr_to_phys(get_cpu_gdt_rw(cpu));
 }
 
+#ifdef CONFIG_X86_64
+/* Provide the current kernel GS base.  */
+static inline void *get_current_kernel_gs_base(void)
+{
+	return this_cpu_ptr(irq_stack_union.gs_base);
+}
+#endif
+
 static inline void pack_gate(gate_desc *gate, unsigned type, unsigned long func,
 			     unsigned dpl, unsigned ist, unsigned seg)
 {
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 842e7c31b53d..a965086c99ce 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1303,16 +1303,6 @@ static inline void kvm_load_ldt(u16 sel)
 	asm("lldt %0" : : "rm"(sel));
 }
 
-#ifdef CONFIG_X86_64
-static inline unsigned long read_msr(unsigned long msr)
-{
-	u64 value;
-
-	rdmsrl(msr, value);
-	return value;
-}
-#endif
-
 static inline u32 get_rdx_init_val(void)
 {
 	return 0x600; /* P6 family */
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index fa998ca8aa5a..83f0ddcf7bfc 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1340,6 +1340,7 @@ static __init int setup_clearcpuid(char *arg)
 #ifdef CONFIG_X86_64
 DEFINE_PER_CPU_FIRST(union irq_stack_union,
 		     irq_stack_union) __aligned(PAGE_SIZE) __visible;
+EXPORT_PER_CPU_SYMBOL_GPL(irq_stack_union);
 
 /*
  * The following percpu variables are hot.  Align current_task to
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 3dbe690c8821..699113cf410e 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1721,7 +1721,7 @@ static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 	}
 
 #ifdef CONFIG_X86_64
-	rdmsrl(MSR_GS_BASE, to_svm(vcpu)->host.gs_base);
+	to_svm(vcpu)->host.gs_base = (u64) get_current_kernel_gs_base();
 #endif
 	savesegment(fs, svm->host.fs);
 	savesegment(gs, svm->host.gs);
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 30e6115d4f09..1ab72ab16b03 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -2188,15 +2188,14 @@ static void vmx_save_host_state(struct kvm_vcpu *vcpu)
 #endif
 
 #ifdef CONFIG_X86_64
-	vmcs_writel(HOST_FS_BASE, read_msr(MSR_FS_BASE));
-	vmcs_writel(HOST_GS_BASE, read_msr(MSR_GS_BASE));
+	vmcs_writel(HOST_FS_BASE, current->thread.fsbase);
+	vmcs_writel(HOST_GS_BASE, (u64) get_current_kernel_gs_base());
 #else
 	vmcs_writel(HOST_FS_BASE, segment_base(vmx->host_state.fs_sel));
 	vmcs_writel(HOST_GS_BASE, segment_base(vmx->host_state.gs_sel));
 #endif
 
 #ifdef CONFIG_X86_64
-	rdmsrl(MSR_KERNEL_GS_BASE, vmx->msr_host_kernel_gs_base);
 	wrmsrl(MSR_KERNEL_GS_BASE, vmx->msr_guest_kernel_gs_base);
 #endif
 	if (boot_cpu_has(X86_FEATURE_MPX))
@@ -2235,7 +2234,7 @@ static void __vmx_load_host_state(struct vcpu_vmx *vmx)
 #endif
 	invalidate_tss_limit();
 #ifdef CONFIG_X86_64
-	wrmsrl(MSR_KERNEL_GS_BASE, vmx->msr_host_kernel_gs_base);
+	wrmsrl(MSR_KERNEL_GS_BASE, current->thread.gsbase);
 #endif
 	if (vmx->host_state.msr_host_bndcfgs)
 		wrmsrl(MSR_IA32_BNDCFGS, vmx->host_state.msr_host_bndcfgs);
@@ -5698,9 +5697,6 @@ static void ept_set_mmio_spte_mask(void)
  */
 static void vmx_vcpu_setup(struct vcpu_vmx *vmx)
 {
-#ifdef CONFIG_X86_64
-	unsigned long a;
-#endif
 	int i;
 
 	if (enable_shadow_vmcs) {
@@ -5749,15 +5745,8 @@ static void vmx_vcpu_setup(struct vcpu_vmx *vmx)
 	vmcs_write16(HOST_FS_SELECTOR, 0);            /* 22.2.4 */
 	vmcs_write16(HOST_GS_SELECTOR, 0);            /* 22.2.4 */
 	vmx_set_constant_host_state(vmx);
-#ifdef CONFIG_X86_64
-	rdmsrl(MSR_FS_BASE, a);
-	vmcs_writel(HOST_FS_BASE, a); /* 22.2.4 */
-	rdmsrl(MSR_GS_BASE, a);
-	vmcs_writel(HOST_GS_BASE, a); /* 22.2.4 */
-#else
 	vmcs_writel(HOST_FS_BASE, 0); /* 22.2.4 */
 	vmcs_writel(HOST_GS_BASE, 0); /* 22.2.4 */
-#endif
 
 	if (cpu_has_vmx_vmfunc())
 		vmcs_write64(VM_FUNCTION_CONTROL, 0);
-- 
1.8.3.1