Re: [RFC PATCH 0/3] generic hypercall support

[Gregory Haskins]

Anthony Liguori wrote:
> Avi Kivity wrote:
>>
>> Hmm, reminds me of something I thought of a while back.
>>
>> We could implement an 'mmio hypercall' that does mmio reads/writes
>> via a hypercall instead of an mmio operation.  That will speed up
>> mmio for emulated devices (say, e1000).  It's easy to hook into Linux
>> (readl/writel), is pci-friendly, non-x86 friendly, etc.
>
> By the time you get down to userspace for an emulated device, that 2us
> difference between mmio and hypercalls is simply not going to make a
> difference.

I don't care about this path for emulated devices.  I am interested in
in-kernel vbus devices.

>   I'm surprised so much effort is going into this, is there any
> indication that this is even close to a bottleneck in any circumstance?

Yes.  Each 1us of overhead is a 4% regression in something as trivial as
a 25us UDP/ICMP rtt "ping".
>
>
> We have much, much lower hanging fruit to attack.  The basic fact that
> we still copy data multiple times in the networking drivers is clearly
> more significant than a few hundred nanoseconds that should occur less
> than once per packet.
for request-response, this is generally for *every* packet since you
cannot exploit buffering/deferring.

Can you back up your claim that PPC has no difference in performance
with an MMIO exit and a "hypercall" (yes, I understand PPC has no "VT"
like instructions, but clearly there are ways to cause a trap, so
presumably we can measure the difference between a PF exit and something
more explicit).

We need numbers before we can really decide to abandon this
optimization.  If PPC mmio has no penalty over hypercall, I am not sure
the 350ns on x86 is worth this effort (especially if I can shrink this
with some RCU fixes).  Otherwise, the margin is quite a bit larger.

-Greg

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