[PATCH RFC 3/3] x86/kvm/vmx: avoid expensive rdmsr for MSR_GS_BASE

[Vitaly Kuznetsov <vkuznets@redhat.com>]

vmx_save_host_state() is only called from kvm_arch_vcpu_ioctl_run() so
the context is pretty well defined and as we're past 'swapgs' MSR_GS_BASE
should contain kernel's GS base which we point to irq_stack_union.

irq_stack_union needs to be exported as KVM can be a module.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 arch/x86/kernel/cpu/common.c | 1 +
 arch/x86/kvm/vmx.c           | 3 ++-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 348cf4821240..057393711093 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1398,6 +1398,7 @@ __setup("clearcpuid=", setup_clearcpuid);
 #ifdef CONFIG_X86_64
 DEFINE_PER_CPU_FIRST(union irq_stack_union,
 		     irq_stack_union) __aligned(PAGE_SIZE) __visible;
+EXPORT_PER_CPU_SYMBOL(irq_stack_union);
 
 /*
  * The following percpu variables are hot.  Align current_task to
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 5be34d13d88f..bf856b7ece3f 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -2158,7 +2158,8 @@ static void vmx_save_host_state(struct kvm_vcpu *vcpu)
 
 #ifdef CONFIG_X86_64
 	vmcs_writel(HOST_FS_BASE, current->thread.fsbase);
-	vmcs_writel(HOST_GS_BASE, read_msr(MSR_GS_BASE));
+	vmcs_writel(HOST_GS_BASE, (unsigned long)
+		    per_cpu(irq_stack_union.gs_base, smp_processor_id()));
 #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));
-- 
2.14.3