KVM: VMX: Only reload guest cr2 if different from host cr2 (d3edefc0) · Commits · 戴 / test

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>

arch/x86/kvm/vmx.c

+7 −2

Original line number	Diff line number	Diff line
		@@ -3651,11 +3651,16 @@ static void vmx_vcpu_run(struct kvm_vcpu vcpu, struct kvm_run kvm_run)
		"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"

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