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

Anthony Liguori
2009-05-08T15:07:19Z

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

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.

Regards,

Anthony Liguori