KVM: VMX: Only reload guest cr2 if different from host cr2
cr2 changes only rarely, and writing it is expensive. Avoid the costly cr2
writes by checking if it does not already hold the desired value.
Shaves 70 cycles off the vmexit latency.
Signed-off-by: Avi Kivity <avi@redhat.com>
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index fc8d49c..1a84ca1 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -3651,11 +3651,16 @@
"mov %%"R"sp, %c[host_rsp](%0) \n\t"
__ex(ASM_VMX_VMWRITE_RSP_RDX) "\n\t"
"1: \n\t"
+ /* Reload cr2 if changed */
+ "mov %c[cr2](%0), %%"R"ax \n\t"
+ "mov %%cr2, %%"R"dx \n\t"
+ "cmp %%"R"ax, %%"R"dx \n\t"
+ "je 2f \n\t"
+ "mov %%"R"ax, %%cr2 \n\t"
+ "2: \n\t"
/* Check if vmlaunch of vmresume is needed */
"cmpl $0, %c[launched](%0) \n\t"
/* Load guest registers. Don't clobber flags. */
- "mov %c[cr2](%0), %%"R"ax \n\t"
- "mov %%"R"ax, %%cr2 \n\t"
"mov %c[rax](%0), %%"R"ax \n\t"
"mov %c[rbx](%0), %%"R"bx \n\t"
"mov %c[rdx](%0), %%"R"dx \n\t"