Re: [patch 1/3] KVM: x86: add ioctls to get/set PIO state

Marcelo Tosatti
2010-02-04T22:45:51Z

On Thu, Feb 04, 2010 at 08:12:07PM -0200, Marcelo Tosatti wrote:
> > >>On 01/28/2010 09:03 PM, Marcelo Tosatti wrote:
> > >>>A vcpu can be stopped after handling IO in userspace,
> > >>>but before returning to kernel to finish processing.
> > >>>
> > >>Is this strictly needed? If we teach qemu to migrate before
> > >>executing the pio request, I think we'll be all right? should work
> > >>at least for IN/INS, not sure about OUT/OUTS.
> > >It would be nice (instead of more state to keep track of between
> > >kernel/user) but the drawbacks i see are:
> > >
> > >You'd have to add a limitation so that any IN which was processed
> > >by device emulation has to re-entry kernel to complete it (so it
> > >complicates vcpu stop in userspace).
> > >
> >
> > You could fix that by moving the IN emulation to before guest entry.
> > It complicates the vcpu loop a bit, but is backwards compatible and
> > all that.
> >
> > >And for OUTS larger than page size (== arch->pio_data size) you need to
> > >know the current position to continue it on the destination (or roll
> > >back the entire effect of the instruction in device emulation, and RIP).
> >
> > What to you mean by current position?
>
> outs larger than PAGE_SIZE is processed in (size / PAGE_SIZE) exits to
> userspace, because thats the size of the pio_data buffer, right?

Nevermind, the count is in ecx which is migrated. OK i'll look into
your suggestion.