Re: [RFC 1/2] virtio_console: Add support for DMA memory allocation

On Thu, Sep 06, 2012 at 08:27:35AM +0300, Michael S. Tsirkin wrote:

On Thu, Sep 06, 2012 at 11:34:25AM +0930, Rusty Russell wrote:

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"Michael S. Tsirkin" [off-list ref] writes:

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On Tue, Sep 04, 2012 at 06:58:47PM +0200, Sjur Brændeland wrote:

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Hi Michael,

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Exactly. Though if we just fail load it will be much less code.

Generally, using a feature bit for this is a bit of a problem though:
normally driver is expected to be able to simply ignore
a feature bit. In this case driver is required to
do something so a feature bit is not a good fit.
I am not sure what the right thing to do is.

I see - so in order to avoid the binding between driver and device
there are two options I guess. Either make virtio_dev_match() or
virtcons_probe() fail. Neither of them seems like the obvious choice.

Maybe adding a check for VIRTIO_CONSOLE_F_DMA_MEM match
between device and driver in virtcons_probe() is the lesser evil?

Regards,
Sjur

A simplest thing to do is change dev id. rusty?

For generic usage, this is correct.  But my opinion is that fallback on
feature non-ack is quality-of-implementation issue: great to have, but
there are cases where you just want to fail with "you're too old".

And in this case, an old system simply will never work.  So it's a
question of how graceful the failure is.

Can your userspace loader can refuse to proceed if the driver doesn't
ack the bits?  If so, it's simpler than a whole new ID.

Cheers,
Rusty.

Yes but how can it signal guest that it will never proceed?

Also grep for BUG_ON in core found this:

       drv->remove(dev);

       /* Driver should have reset device. */
       BUG_ON(dev->config->get_status(dev));

I think below is what Sjur refers to.
I think below is a good idea for 3.6. Thoughts?

--->

virtio: don't crash when device is buggy

Because of a sanity check in virtio_dev_remove, a buggy device can crash
kernel.  And in case of rproc it's userspace so it's not a good idea.
We are unloading a driver so how bad can it be?
Be less aggressive in handling this error: if it's a driver bug,
warning once should be enough.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

--

Rusty?