Re: [PATCH net-next v4 2/2] virtio-net: add cond_resched() to the command waiting loop

[Jason Wang]

On Fri, Aug 11, 2023 at 3:41 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Tue, Aug 08, 2023 at 10:30:56AM +0800, Jason Wang wrote:
> > On Mon, Jul 31, 2023 at 2:30 PM Jason Wang <jasowang@xxxxxxxxxx> wrote:
> > >
> > > On Thu, Jul 27, 2023 at 5:46 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> > > >
> > > > On Thu, Jul 27, 2023 at 04:59:33PM +0800, Jason Wang wrote:
> > > > > > They really shouldn't - any NIC that takes forever to
> > > > > > program will create issues in the networking stack.
> > > > >
> > > > > Unfortunately, it's not rare as the device/cvq could be implemented
> > > > > via firmware or software.
> > > >
> > > > Currently that mean one either has sane firmware with a scheduler that
> > > > can meet deadlines, or loses ability to report errors back.
> > > >
> > > > > > But if they do they can always set this flag too.
> > > > >
> > > > > This may have false negatives and may confuse the management.
> > > > >
> > > > > Maybe we can extend the networking core to allow some device specific
> > > > > configurations to be done with device specific lock without rtnl. For
> > > > > example, split the set_channels to
> > > > >
> > > > > pre_set_channels
> > > > > set_channels
> > > > > post_set_channels
> > > > >
> > > > > The device specific part could be done in pre and post without a rtnl lock?
> > > > >
> > > > > Thanks
> > > >
> > > >
> > > > Would the benefit be that errors can be reported to userspace then?
> > > > Then maybe.  I think you will have to show how this works for at least
> > > > one card besides virtio.
> > >
> > > Even for virtio, this seems not easy, as e.g the
> > > virtnet_send_command() and netif_set_real_num_tx_queues() need to
> > > appear to be atomic to the networking core.
> > >
> > > I wonder if we can re-consider the way of a timeout here and choose a
> > > sane value as a start.
> >
> > Michael, any more input on this?
> >
> > Thanks
>
> I think this is just mission creep. We are trying to fix
> vduse - let's do that for starters.
>
> Recovering from firmware timeouts is far from trivial and
> just assuming that just because it timed out it will not
> access memory is just as likely to cause memory corruption
> with worse results than an infinite spin.

Yes, this might require support not only in the driver

>
> I propose we fix this for vduse and assume hardware/firmware
> is well behaved.

One major case is the re-connection, in that case it might take
whatever longer that the kernel virito-net driver expects.

So we can have a timeout in VDUSE and trap CVQ then VDUSE can return
and fail early?

> Or maybe not well behaved firmware will
> set the flag losing error reporting ability.

This might be hard since it means not only the set but also the get is
unreliable.

Thanks

>
>
>
> > >
> > > Thanks
> > >
> > > >
> > > >
> > > > --
> > > > MST
> > > >
>