From f6577a5fa15d82217ca73c74cd2dcbc0f6c781dd Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Fri, 7 Nov 2014 18:25:18 -0800 Subject: [PATCH] x86, kvm, vmx: Always use LOAD_IA32_EFER if available 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 [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] Signed-off-by: Paolo Bonzini --- 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 284f5c2fdf0b..0d25fc16d98b 100644 --- a/arch/x86/kvm/vmx.c +++ b/arch/x86/kvm/vmx.c @@ -1662,8 +1662,14 @@ 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; -- 2.20.1