Re: Seeking a KVM benchmark

Andy Lutomirski
2014-11-08T16:45:04Z

On Sat, Nov 8, 2014 at 8:00 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Nov 8, 2014 4:01 AM, "Gleb Natapov" <gleb@kernel.org> wrote:
>>
>> On Fri, Nov 07, 2014 at 09:59:55AM -0800, Andy Lutomirski wrote:
>> > On Thu, Nov 6, 2014 at 11:17 PM, Paolo Bonzini <pbonzini@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?
>
> I don't understand. AFAICT there are really only two cases: EFER
> switched atomically using the best available mechanism on the host
> CPU, or EFER switched on userspace exit. I think there's a
> theoretical third possibility: if the guest and host EFER match, then
> EFER doesn't need to be switched at all, but this doesn't seem to be
> implemented.

I got this part wrong. It looks like the user return notifier is
smart enough not to set EFER at all if the guest and host values
match. Indeed, with stock KVM, if I modify vmexit.c to have exactly
the same EFER as the host (NX and SCE both set), then it runs quickly.
But I get almost exactly the same performance if NX is clear, which is
the case where the built-in entry/exit switching is used.

Admittedly, most guests probably do match the host, so this effect may
be rare in practice. But possibly the code should be changed either
the way I patched it (always use the built-in switching if available)
or to only do it if the guest and host EFER values differ. ISTM that,
on modern CPUs, switching EFER on return to userspace is always a big
loss.

If neither change is made, then maybe the test should change to set
SCE so that it isn't so misleadingly slow.

--Andy

>
>>
>> Your change reduced userspace exit cost by ~30%, but what about exit to kernel?
>> We have much more of those.
>
> My KVM patch to change the heuristic didn't seem to slow down kernel
> exits at all. In fact, it got faster, but possibly not significantly.
> This makes me suspect that the newer EFER entry/exit controls are
> actually free. This wouldn't surprise me all that much, since the
> microcode has to fiddle with LME and such anyway, and just switching
> the whole register could be easier than thinking about which bits to
> switch.
>
> My KVM patch and actual benchmarks are here:
>
> http://article.gmane.org/gmane.linux.kernel/1824469
>
> I used the wrong email address for you, and it doesn't seem to have
> made it to the KVM list, though.
>
> --Andy

--
Andy Lutomirski
AMA Capital Management, LLC