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

Avi Kivity
2009-05-08T18:51:20Z

Anthony Liguori wrote:
>
> And we're now getting close to the point where the difference is
> virtually meaningless.
>
> At .14us, in order to see 1% CPU overhead added from PIO vs HC, you
> need 71429 exits.
>

If I read things correctly, you want the difference between PIO and
PIOoHC, which is 210ns. But your point stands, 50,000 exits/sec will
add 1% cpu overhead.

>
> The non-x86 architecture argument isn't valid because other
> architectures either 1) don't use PCI at all (s390) and are already
> using hypercalls 2) use PCI, but do not have a dedicated hypercall
> instruction (PPC emb) or 3) have PIO (ia64).

ia64 uses mmio to emulate pio, so the cost may be different. I agree on
x86 it's almost negligible.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.