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

[Anthony Liguori]

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

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