x86/kvm/vmx: avoid expensive rdmsr for MSR_GS_BASE

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.

Add new kernelmode_gs_base() API, irq_stack_union needs to be exported
as KVM can be build as module.
Acked-by: NAndy Lutomirski <luto@kernel.org>
Signed-off-by: NVitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/processor.h

@@ -407,6 +407,11 @@ union irq_stack_union {
 DECLARE_PER_CPU_FIRST(union irq_stack_union, irq_stack_union) __visible;
 DECLARE_INIT_PER_CPU(irq_stack_union);
 
+static inline unsigned long cpu_kernelmode_gs_base(int cpu)
+{
+	return (unsigned long)per_cpu(irq_stack_union.gs_base, cpu);
+}
+
 DECLARE_PER_CPU(char *, irq_stack_ptr);
 DECLARE_PER_CPU(unsigned int, irq_count);
 extern asmlinkage void ignore_sysret(void);

arch/x86/kernel/cpu/common.c

@@ -487,7 +487,7 @@ void load_percpu_segment(int cpu)
 	loadsegment(fs, __KERNEL_PERCPU);
 #else
 	__loadsegment_simple(gs, 0);
-	wrmsrl(MSR_GS_BASE, (unsigned long)per_cpu(irq_stack_union.gs_base, cpu));
+	wrmsrl(MSR_GS_BASE, cpu_kernelmode_gs_base(cpu));
 #endif
 	load_stack_canary_segment();
 }
@@ -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_GPL(irq_stack_union);
 
 /*
  * The following percpu variables are hot.  Align current_task to

arch/x86/kvm/vmx.c

@@ -2153,6 +2153,7 @@ static unsigned long segment_base(u16 selector)
 static void vmx_save_host_state(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
+	int cpu = raw_smp_processor_id();
 	int i;
 
 	if (vmx->host_state.loaded)
@@ -2195,7 +2196,7 @@ 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, cpu_kernelmode_gs_base(cpu));
 #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));