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Commit 4d56c8a7 authored by Avi Kivity's avatar Avi Kivity
Browse files

KVM: VMX: Only save/restore MSR_K6_STAR if necessary 



Intel hosts only support syscall/sysret in long more (and only if efer.sce
is enabled), so only reload the related MSR_K6_STAR if the guest will
actually be able to use it.

This reduces vmexit cost by about 500 cycles (6400 -> 5870) on my setup.

Signed-off-by: default avatarAvi Kivity <avi@qumranet.com>
parent 35cc7f97

drivers/kvm/vmx.c

+16 −0
Original line number Diff line number Diff line
@@ -69,6 +69,10 @@ static struct kvm_vmx_segment_field { 
	VMX_SEGMENT_FIELD(LDTR),

};



/*

 * Keep MSR_K6_STAR at the end, as setup_msrs() will try to optimize it

 * away by decrementing the array size.

 */

static const u32 vmx_msr_index[] = {

#ifdef CONFIG_X86_64

	MSR_SYSCALL_MASK, MSR_LSTAR, MSR_CSTAR, MSR_KERNEL_GS_BASE,
@@ -323,6 +327,18 @@ static void setup_msrs(struct kvm_vcpu *vcpu) 
		nr_skip = NR_64BIT_MSRS;

	nr_good_msrs = vcpu->nmsrs - nr_skip;



	/*

	 * MSR_K6_STAR is only needed on long mode guests, and only

	 * if efer.sce is enabled.

	 */

	if (find_msr_entry(vcpu, MSR_K6_STAR)) {

		--nr_good_msrs;

#ifdef CONFIG_X86_64

		if (is_long_mode(vcpu) && (vcpu->shadow_efer & EFER_SCE))

			++nr_good_msrs;

#endif

	}



	vmcs_writel(VM_ENTRY_MSR_LOAD_ADDR,

		    virt_to_phys(vcpu->guest_msrs + nr_skip));

	vmcs_writel(VM_EXIT_MSR_STORE_ADDR,