On Fri, Nov 07, 2014 at 09:59:55AM -0800, Andy Lutomirski wrote:
> On Thu, Nov 6, 2014 at 11:17 PM, Paolo Bonzini <pbonz...@redhat.com> wrote:
> >
> >
> > On 07/11/2014 07:27, Andy Lutomirski wrote:
> >> Is there an easy benchmark that's sensitive to the time it takes to
> >> round-trip from userspace to guest and back to userspace? I think I
> >> may have a big speedup.
> >
> > The simplest is vmexit.flat from
> > git://git.kernel.org/pub/scm/virt/kvm/kvm-unit-tests.git
> >
> > Run it with "x86/run x86/vmexit.flat" and look at the inl_from_qemu
> > benchmark.
>
> Thanks!
>
> That test case is slower than I expected. I think my change is likely
> to save somewhat under 100ns, which is only a couple percent. I'll
> look for more impressive improvements.
>
> On a barely related note, in the process of poking around with this
> test, I noticed:
>
> /* On ept, can't emulate nx, and must switch nx atomically */
> if (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;
> add_atomic_switch_msr(vmx, MSR_EFER, guest_efer, host_efer);
> return false;
> }
>
> return true;
>
> This heuristic seems wrong to me. wrmsr is serializing and therefore
> extremely slow, whereas I imagine that, on CPUs that support it,
> atomically switching EFER ought to be reasonably fast.
>
> Indeed, changing vmexit.c to disable NX (thereby forcing atomic EFER
> switching, and having no other relevant effect that I've thought of)
> speeds up inl_from_qemu by ~30% on Sandy Bridge. Would it make sense
> to always use atomic EFER switching, at least when
> cpu_has_load_ia32_efer?
>
The idea behind current logic is that we want to avoid writing an MSR
at all for lightweight exists (those that do not exit to userspace). So
if NX bit is the same for host and guest we can avoid writing EFER on
exit and run with guest's EFER in the kernel. But if userspace exit is
required only then we write host's MSR back, only if guest and host MSRs
are different of course. What bit should be restored on userspace exit
in vmexit tests? Is it SCE? What if you set it instead of unsetting NXE?
Your change reduced userspace exit cost by ~30%, but what about exit to kernel?
We have much more of those.
--
Gleb.
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