Re: [Qemu-devel] [PATCH] migration: avoid copying ignore-shared ramblock when in incoming migration

[Catherine Ho]

On Tue, 2 Apr 2019 at 15:47, Catherine Ho <address@hidden> wrote:

> Hi Peter Maydell
>
> On Tue, 2 Apr 2019 at 11:05, Peter Maydell <address@hidden>
> wrote:
>
>> On Tue, 2 Apr 2019 at 09:57, Catherine Ho <address@hidden>
>> wrote:
>> > The root cause is the used idx is moved forward after 1st time
>> incoming, and in 2nd time incoming,
>> > the last_avail_idx will be incorrectly restored from the saved device
>> state file(not in the ram).
>> >
>> > I watched this even on x86 for a virtio-scsi disk
>> >
>> > Any ideas for supporting 2nd time, 3rd time... incoming restoring?
>>
>> Does the destination end go through reset between the 1st and 2nd
>>
> seems not, please see my step below
>
>> incoming attempts? I'm not a migration expert, but I thought that
>> devices were allowed to assume that their state is "state of the
>> device following QEMU reset" before the start of an incoming
>> migration attempt.
>>
>
> Here  is my step:
> 1. start guest normal by qemu with shared memory-backend file
> 2. stop the vm. save the device state to another file via monitor migrate
> "exec: cat>..."
> 3. quit the vm
>
via "quit" command of monitor

> 4. retore the vm by qemu -incoming "exec:cat ..."
> 5. continue the vm via monito, the 1st incoming works fine
> 6. quit the vm
> 7. retore the vm by qemu -incoming "exec:cat ..." for 2nd time
> 8. continue   -> error happened
> Actually, this can be fixed by forcely restore the idx by
> virtio_queue_restore_last_avail_idx(）
> But I am sure whether it is reasonable.
>
s/sure/not sure

>
> B.R.
>
>>
>> thanks
>> -- PMM
>>
>