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SubjectRe: [PATCH] x86, kvm, vmx: Always use LOAD_IA32_EFER if available


On 08/11/2014 03:25, Andy Lutomirski wrote:
> At least on Sandy Bridge, letting the CPU switch IA32_EFER is much
> faster than switching it manually.
>
> I benchmarked this using the vmexit kvm-unit-test (single run, but
> GOAL multiplied by 5 to do more iterations):
>
> Test Before After Change
> cpuid 2000 1932 -3.40%
> vmcall 1914 1817 -5.07%
> mov_from_cr8 13 13 0.00%
> mov_to_cr8 19 19 0.00%
> inl_from_pmtimer 19164 10619 -44.59%
> inl_from_qemu 15662 10302 -34.22%
> inl_from_kernel 3916 3802 -2.91%
> outl_to_kernel 2230 2194 -1.61%
> mov_dr 172 176 2.33%
> ipi (skipped) (skipped)
> ipi+halt (skipped) (skipped)
> ple-round-robin 13 13 0.00%
> wr_tsc_adjust_msr 1920 1845 -3.91%
> rd_tsc_adjust_msr 1892 1814 -4.12%
> mmio-no-eventfd:pci-mem 16394 11165 -31.90%
> mmio-wildcard-eventfd:pci-mem 4607 4645 0.82%
> mmio-datamatch-eventfd:pci-mem 4601 4610 0.20%
> portio-no-eventfd:pci-io 11507 7942 -30.98%
> portio-wildcard-eventfd:pci-io 2239 2225 -0.63%
> portio-datamatch-eventfd:pci-io 2250 2234 -0.71%
>
> I haven't explicitly computed the significance of these numbers,
> but this isn't subtle.
>
> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> ---
> arch/x86/kvm/vmx.c | 10 ++++++++--
> 1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 3e556c68351b..e72b9660e51c 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -1659,8 +1659,14 @@ static bool update_transition_efer(struct vcpu_vmx *vmx, int efer_offset)
> vmx->guest_msrs[efer_offset].mask = ~ignore_bits;
>
> clear_atomic_switch_msr(vmx, MSR_EFER);
> - /* On ept, can't emulate nx, and must switch nx atomically */
> - if (enable_ept && ((vmx->vcpu.arch.efer ^ host_efer) & EFER_NX)) {
> +
> + /*
> + * On EPT, we can't emulate NX, so we must switch EFER atomically.
> + * On CPUs that support "load IA32_EFER", always switch EFER
> + * atomically, since it's faster than switching it manually.
> + */
> + if (cpu_has_load_ia32_efer ||
> + (enable_ept && ((vmx->vcpu.arch.efer ^ host_efer) & EFER_NX))) {
> guest_efer = vmx->vcpu.arch.efer;
> if (!(guest_efer & EFER_LMA))
> guest_efer &= ~EFER_LME;
>

I am committing this patch, with an additional remark in the commit message:

The results were reproducible on all of Nehalem, Sandy Bridge and
Ivy Bridge. The slowness of manual switching is because writing
to EFER with WRMSR triggers a TLB flush, even if the only bit you're
touching is SCE (so the page table format is not affected). Doing
the write as part of vmentry/vmexit, instead, does not flush the TLB,
probably because all processors that have EPT also have VPID.

Paolo


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