Re: [PATCH] tools/libxenguest: Fix migration's debug option

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.07.2021 12:06, Andrew Cooper wrote:
> On 05/07/2021 10:31, Jan Beulich wrote:
>> On 05.07.2021 11:25, Olaf Hering wrote:
>>> Am Mon, 5 Jul 2021 11:19:59 +0200
>>> schrieb Jan Beulich <jbeulich@xxxxxxxx>:
>>>
>>>> "The interface" being which one? The tool stack can map the guest's
>>>> grant table, so it is in the position to find out about all grants
>>>> without further hypervisor help.
>>> The interface means the code behind verify_frames.
>>>
>>> If there are indeed ways to query which pages belong to grants, how would 
>>> the toolstack need to do that?
>> Map the grant table of the guest and walk it, recording any MFN for
>> which at least one valid r/w grant exists.
> 
> That doesn't help - Its still racy with in-flight IO.

Well, I meant the recorded data to be used to simply not verify
those frames.

>  Also with updates from Xen such as the wallclocks.

This doesn't occur for a paused domain, does it?

> The only way to fix the IO problem is to disconnect the blk/net rings
> before doing the final sweep for frames, but that clobbers any ability
> to restart the VM on the source side if things go wrong at the destination.
> 
> I don't have an answer at all for the vcpu info frames.

Yeah, they fall in the same category as the FIFO control pages, as
they contain evtchn_{pending_sel,upcall_pending}.

Jan