Re: [PATCH] xen-hvm: Avoid livelock while handling buffered ioreqs

[Paul Durrant <xadimgnik@gmail.com>]

On 08/04/2024 14:00, Ross Lagerwall wrote:
> On Sat, Apr 6, 2024 at 11:58 AM Durrant, Paul <xadimgnik@gmail.com> wrote:
>>
>> On 04/04/2024 15:08, Ross Lagerwall wrote:
>>> A malicious or buggy guest may generated buffered ioreqs faster than
>>> QEMU can process them in handle_buffered_iopage(). The result is a
>>> livelock - QEMU continuously processes ioreqs on the main thread without
>>> iterating through the main loop which prevents handling other events,
>>> processing timers, etc. Without QEMU handling other events, it often
>>> results in the guest becoming unsable and makes it difficult to stop the
>>> source of buffered ioreqs.
>>>
>>> To avoid this, if we process a full page of buffered ioreqs, stop and
>>> reschedule an immediate timer to continue processing them. This lets
>>> QEMU go back to the main loop and catch up.
>>>
>>
>> Do PV backends potentially cause the same scheduling issue (if not using
>> io threads)?
>>
> 
>  From what I can tell:
> 
> xen-block: It reads req_prod / req_cons once before entering the loop
> so it should be fine, I think.
> 
> xen_console: Same as xen-block
> 
> xen_nic: It reads req_prod / req_cons once before entering the loop.
> However, once the loop ends it checks for more requests and if there
> are more requests it restarts from the beginning. It seems like this
> could be susceptible to the same issue.
> 
> (These PV backends generally aren't used by XenServer's system QEMU
> so I didn't spend too much time looking into it.)
> 
> Thanks,

Ok. Thanks for checking.

   Paul