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KVM: VMX: Only save/restore MSR_K6_STAR if necessary

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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: Avi Kivity <avi@qumranet.com>
This commit is contained in:
Avi Kivity 2007-04-19 14:28:44 +03:00
parent 35cc7f9711
commit 4d56c8a787
1 changed files with 16 additions and 0 deletions
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16
drivers/kvm/vmx.c
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@ -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,