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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Feb 04, 2010 at 11:46:25PM +0200, Avi Kivity wrote:
> On 02/04/2010 11:36 PM, Marcelo Tosatti wrote:
> >On Thu, Feb 04, 2010 at 09:16:47PM +0200, Avi Kivity 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?