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

Marcelo Tosatti
2010-02-04T21:37:07Z

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).

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).