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

Arnd Bergmann
2009-05-07T20:43:32Z

On Thursday 07 May 2009, Gregory Haskins wrote:
> Arnd Bergmann wrote:
> > An mmio that goes through a PF is a bug, it's certainly broken on
> > a number of platforms, so performance should not be an issue there.
> >
>
> This may be my own ignorance, but I thought a VMEXIT of type "PF" was
> how MMIO worked in VT/SVM.

You are right that all MMIOs (and PIO on most non-x86 architectures)
are handled this way in the end. What I meant was that an MMIO that
traps because of a simple pointer dereference as in __raw_writel
is a bug, while any actual writel() call could be diverted to
do an hcall and therefore not cause a PF once the infrastructure
is there.

> I guess the problem that was later pointed out is that we cannot discern
> which devices might be pass-through and therefore should not be
> revectored through a HC. But I am even less knowledgeable about how
> pass-through works than I am about the MMIO traps, so I might be
> completely off here.

An easy way to deal with the pass-through case might be to actually use
__raw_writel there. In guest-to-guest communication, the two sides are
known to have the same endianess (I assume) and you can still add the
appropriate smp_mb() and such into the code.