Subject: Re: [PATCH] virtio-net: Read MAC only after initializing MSI-X

From: Sasha Levin <[email-masked]>
Date: Sun, 14 Aug 2011 16:57:32 +0300

On Sun, 2011-08-14 at 12:23 +0930, Rusty Russell wrote:
> On Sat, 13 Aug 2011 11:51:01 +0300, Sasha Levin <[email-masked]> wrote:
> > The MAC of a virtio-net device is located at the first field of the device
> > specific header. This header is located at offset 20 if the device doesn't
> > support MSI-X or offset 24 if it does.
> 
> Erk.  This means, in general, we have to do virtio_find_single_vq or
> config->find_vqs before we examine any config options.
> 
> Look at virtio_blk, which has the same error.
> 
> Solutions in order of best to worst:
> (1) Enable MSI-X before calling device probe.  This means reserving two
>     vectors in virtio_pci_probe to ensure we *can* do this, I think.  Michael?

Do you mean reserving the vectors even before we probed the device for
MSI-X support? Wouldn't we need 3 vectors then? (config, input, output).

> (2) Ensure ordering of "find_vqs then access config space" statically.  This
>     probably means handing the vqs array to virtio_config_val, so noone
>     can call it before they have their virtqueues.

Just noticed that only virtio-blk uses virtio_config_val(), while the
others are still doing 'if(virtio_has_feature()) vdev->config->get()',
I'll send patches to fix that regardless of what we end up doing here.

Did you want to pass the vq array to virtio_config_val() just to check
that they were already found? 

> (3) Ensure ordering dynamically, ie. BUG_ON() if they haven't done
>     find_vqs when they call the config accessors.
> 
> If (1) is too invasive for -stable, then we should rearrange the drivers
> in separate patches (and cc: -stable), then fix it properly.
> 
> Good catch Sasha!
> 
> Cheers,
> Rusty.

-- 

Sasha.