Re: [PATCH RFC] kvm: add PV MMIO EVENTFD

[Michael S. Tsirkin]

On Thu, Apr 04, 2013 at 03:06:42PM +0200, Alexander Graf wrote:
> 
> On 04.04.2013, at 14:56, Gleb Natapov wrote:
> 
> > On Thu, Apr 04, 2013 at 02:49:39PM +0200, Alexander Graf wrote:
> >> 
> >> On 04.04.2013, at 14:45, Gleb Natapov wrote:
> >> 
> >>> On Thu, Apr 04, 2013 at 02:39:51PM +0200, Alexander Graf wrote:
> >>>> 
> >>>> On 04.04.2013, at 14:38, Gleb Natapov wrote:
> >>>> 
> >>>>> On Thu, Apr 04, 2013 at 02:32:08PM +0200, Alexander Graf wrote:
> >>>>>> 
> >>>>>> On 04.04.2013, at 14:08, Gleb Natapov wrote:
> >>>>>> 
> >>>>>>> On Thu, Apr 04, 2013 at 01:57:34PM +0200, Alexander Graf wrote:
> >>>>>>>> 
> >>>>>>>> On 04.04.2013, at 12:50, Michael S. Tsirkin wrote:
> >>>>>>>> 
> >>>>>>>>> With KVM, MMIO is much slower than PIO, due to the need to
> >>>>>>>>> do page walk and emulation. But with EPT, it does not have to be: we
> >>>>>>>>> know the address from the VMCS so if the address is unique, we can look
> >>>>>>>>> up the eventfd directly, bypassing emulation.
> >>>>>>>>> 
> >>>>>>>>> Add an interface for userspace to specify this per-address, we can
> >>>>>>>>> use this e.g. for virtio.
> >>>>>>>>> 
> >>>>>>>>> The implementation adds a separate bus internally. This serves two
> >>>>>>>>> purposes:
> >>>>>>>>> - minimize overhead for old userspace that does not use PV MMIO
> >>>>>>>>> - minimize disruption in other code (since we don't know the length,
> >>>>>>>>> devices on the MMIO bus only get a valid address in write, this
> >>>>>>>>> way we don't need to touch all devices to teach them handle
> >>>>>>>>> an dinvalid length)
> >>>>>>>>> 
> >>>>>>>>> At the moment, this optimization is only supported for EPT on x86 and
> >>>>>>>>> silently ignored for NPT and MMU, so everything works correctly but
> >>>>>>>>> slowly.
> >>>>>>>>> 
> >>>>>>>>> TODO: NPT, MMU and non x86 architectures.
> >>>>>>>>> 
> >>>>>>>>> The idea was suggested by Peter Anvin.  Lots of thanks to Gleb for
> >>>>>>>>> pre-review and suggestions.
> >>>>>>>>> 
> >>>>>>>>> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> >>>>>>>> 
> >>>>>>>> This still uses page fault intercepts which are orders of magnitudes slower than hypercalls. Why don't you just create a PV MMIO hypercall that the guest can use to invoke MMIO accesses towards the host based on physical addresses with explicit length encodings?
> >>>>>>>> 
> >>>>>>> It is slower, but not an order of magnitude slower. It become faster
> >>>>>>> with newer HW.
> >>>>>>> 
> >>>>>>>> That way you simplify and speed up all code paths, exceeding the speed of PIO exits even. It should also be quite easily portable, as all other platforms have hypercalls available as well.
> >>>>>>>> 
> >>>>>>> We are trying to avoid PV as much as possible (well this is also PV,
> >>>>>>> but not guest visible
> >>>>>> 
> >>>>>> Also, how is this not guest visible? Who sets KVM_IOEVENTFD_FLAG_PV_MMIO? The comment above its definition indicates that the guest does so, so it is guest visible.
> >>>>>> 
> >>>>> QEMU sets it.
> >>>> 
> >>>> How does QEMU know?
> >>>> 
> >>> Knows what? When to create such eventfd? virtio device knows.
> >> 
> >> Where does it know from?
> >> 
> > It does it always.
> > 
> >>> 
> >>>>> 
> >>>>>> +/*
> >>>>>> + * PV_MMIO - Guest can promise us that all accesses touching this address
> >>>>>> + * are writes of specified length, starting at the specified address.
> >>>>>> + * If not - it's a Guest bug.
> >>>>>> + * Can not be used together with either PIO or DATAMATCH.
> >>>>>> + */
> >>>>>> 
> >>>>> Virtio spec will state that access to a kick register needs to be of
> >>>>> specific length. This is reasonable thing for HW to ask.
> >>>> 
> >>>> This is a spec change. So the guest would have to indicate that it adheres to a newer spec. Thus it's a guest visible change.
> >>>> 
> >>> There is not virtio spec that has kick register in MMIO. The spec is in
> >>> the works AFAIK. Actually PIO will not be deprecated and my suggestion
> >> 
> >> So the guest would indicate that it supports a newer revision of the spec (in your case, that it supports MMIO). How is that any different from exposing that it supports a PV MMIO hcall?
> >> 
> > Guest will indicate nothing. New driver will use MMIO if PIO is bar is
> > not configured. All driver will not work for virtio devices with MMIO
> > bar, but not PIO bar.
> 
> I can't parse that, sorry :).

It's simple. Driver does iowrite16 or whatever is appropriate for the OS.
QEMU tells KVM which address driver uses, to make exits faster.  This is not
different from how eventfd works.  For example if exits to QEMU suddenly become
very cheap we can remove eventfd completely.

> > 
> >>> is to move to MMIO only when PIO address space is exhausted. For PCI it
> >>> will be never, for PCI-e it will be after ~16 devices.
> >> 
> >> Ok, let's go back a step here. Are you actually able to measure any speed in performance with this patch applied and without when going through MMIO kicks?
> >> 
> >> 
> > That's the question for MST. I think he did only micro benchmarks till
> > now and he already posted his result here:
> > 
> > mmio-wildcard-eventfd:pci-mem 3529
> > mmio-pv-eventfd:pci-mem 1878
> > portio-wildcard-eventfd:pci-io 1846
> > 
> > So the patch speedup mmio by almost 100% and it is almost the same as PIO.
> 
> Those numbers don't align at all with what I measured.

Yep. But why?
Could be a different hardware. My laptop is i7, what did you measure on?
processor       : 0
vendor_id       : GenuineIntel
cpu family      : 6
model           : 42
model name      : Intel(R) Core(TM) i7-2640M CPU @ 2.80GHz
stepping        : 7
microcode       : 0x28
cpu MHz         : 2801.000
cache size      : 4096 KB

Or could be different software, this is on top of 3.9.0-rc5, what
did you try?

> MST, could you please do a real world latency benchmark with virtio-net and
> 
>   * normal ioeventfd
>   * mmio-pv eventfd
>   * hcall eventfd

I can't do this right away, sorry.  For MMIO we are discussing the new
layout on the virtio mailing list, guest and qemu need a patch for this
too.  My hcall patches are stale and would have to be brought up to
date.


> to give us some idea how much performance we would gain from each approach? Thoughput should be completely unaffected anyway, since virtio just coalesces kicks internally.

Latency is dominated by the scheduling latency.
This means virtio-net is not the best benchmark.

> I'm also slightly puzzled why the wildcard eventfd mechanism is so significantly slower, while it was only a few percent on my test system. What are the numbers you're listing above? Cycles? How many cycles do you execute in a second?
> 
> 
> Alex


It's the TSC divided by number of iterations.  kvm unittest this value, here's
what it does (removed some dead code):

#define GOAL (1ull << 30)

        do {
                iterations *= 2;
                t1 = rdtsc();

                        for (i = 0; i < iterations; ++i)
                                func();
                t2 = rdtsc();
        } while ((t2 - t1) < GOAL);
        printf("%s %d\n", test->name, (int)((t2 - t1) / iterations));


-- 
MST

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