Re: [PATCH V5 00/22] IOREQ feature (+ virtio-mmio) on Arm

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 28 Jan 2021, Julien Grall wrote:
> On 25/01/2021 19:08, Oleksandr Tyshchenko wrote:
>  > Patch series [8] was rebased on recent "staging branch"
> > (5e31789 tools/ocaml/libs/xb: Do not crash after xenbus is unmapped) and
> > tested on
> > Renesas Salvator-X board + H3 ES3.0 SoC (Arm64) with virtio-mmio disk
> > backend [9]
> > running in driver domain and unmodified Linux Guest running on existing
> > virtio-blk driver (frontend). No issues were observed. Guest domain
> > 'reboot/destroy'
> > use-cases work properly. Patch series was only build-tested on x86.
> 
> I have done basic testing with a Linux guest on x86 and I didn't spot any
> regression.
> 
> Unfortunately, I don't have a Windows VM in hand, so I can't confirm if there
> is no regression there. Can anyone give a try?
> 
> On a separate topic, Andrew expressed on IRC some concern to expose the
> bufioreq interface to other arch than x86. I will let him explain his view
> here.
> 
> Given that IOREQ will be a tech preview on Arm (this implies the interface is
> not stable) on Arm, I think we can defer the discussion past the freeze.

Yes, I agree that we can defer the discussion.


> For now, I would suggest to check if a Device Model is trying to create an
> IOREQ server with bufioreq is enabled (see ioreq_server_create()). This would
> not alleviate Andrew's concern, however it would at allow us to judge whether
> the buffering concept is going to be used on Arm (I can see some use for the
> Virtio doorbell).
> 
> What do others think?

Difficult to say. We don't have any uses today but who knows in the
future.

Maybe for now the important thing is to clarify that the bufioreq
interface won't be maintain for backward compatibility on ARM, and could
be removed without warnings, at least as long as the whole IOREQ feature
is a tech preview.

In other words, it is not like we are forced to keep bufioreq around
forever on ARM.