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

Avi Kivity
2010-02-04T21:46:30Z

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?

--
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signature is too narrow to contain.