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

Anthony Liguori
2009-05-08T19:02:24Z

Avi Kivity wrote:
> 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.

Right, the basic math still stands.

>>
>> 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.

Yes, I misunderstood that they actually emulated it like that. However,
ia64 has no paravirtualization support today so surely, we aren't going
to be justifying this via ia64, right?

Regards,

Anthony Liguori