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Subject[PATCH] KVM: x86: do not read FS/GS base MSRs when saving them
Date
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@redhat.com>
---
arch/x86/include/asm/desc.h | 6 ++++++
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, 11 insertions(+), 25 deletions(-)

diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
index 4011cb03ef08..5a9a20b74770 100644
--- a/arch/x86/include/asm/desc.h
+++ b/arch/x86/include/asm/desc.h
@@ -85,6 +85,12 @@ static inline phys_addr_t get_cpu_gdt_paddr(unsigned int cpu)
return per_cpu_ptr_to_phys(get_cpu_gdt_rw(cpu));
}

+/* Provide the current kernel GS base. */
+static inline void *get_current_kernel_gs_base(void)
+{
+ return this_cpu_ptr(irq_stack_union.gs_base);
+}
+
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
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