KVM: VMX: Avoid vmwrite(HOST_RSP) when possible

Usually HOST_RSP retains its value across guest entries.  Take advantage
of this and avoid a vmwrite() when this is so.
Signed-off-by: Avi Kivity <avi@qumranet.com>

arch/x86/kvm/vmx.c

@@ -58,6 +58,7 @@ struct vmcs {
 struct vcpu_vmx {
 	struct kvm_vcpu       vcpu;
 	struct list_head      local_vcpus_link;
+	unsigned long         host_rsp;
 	int                   launched;
 	u8                    fail;
 	u32                   idt_vectoring_info;
@@ -2982,7 +2983,11 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 		/* Store host registers */
 		"push %%"R"dx; push %%"R"bp;"
 		"push %%"R"cx \n\t"
+		"cmp %%"R"sp, %c[host_rsp](%0) \n\t"
+		"je 1f \n\t"
+		"mov %%"R"sp, %c[host_rsp](%0) \n\t"
 		__ex(ASM_VMX_VMWRITE_RSP_RDX) "\n\t"
+		"1: \n\t"
 		/* Check if vmlaunch of vmresume is needed */
 		"cmpl $0, %c[launched](%0) \n\t"
 		/* Load guest registers.  Don't clobber flags. */
@@ -3039,6 +3044,7 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 	      : : "c"(vmx), "d"((unsigned long)HOST_RSP),
 		[launched]"i"(offsetof(struct vcpu_vmx, launched)),
 		[fail]"i"(offsetof(struct vcpu_vmx, fail)),
+		[host_rsp]"i"(offsetof(struct vcpu_vmx, host_rsp)),
 		[rax]"i"(offsetof(struct vcpu_vmx, vcpu.arch.regs[VCPU_REGS_RAX])),
 		[rbx]"i"(offsetof(struct vcpu_vmx, vcpu.arch.regs[VCPU_REGS_RBX])),
 		[rcx]"i"(offsetof(struct vcpu_vmx, vcpu.arch.regs[VCPU_REGS_RCX])),