linux-kernel - Re: [PATCH v3 04/16] virtio/console: verify device has config space

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Message-ID: <20150121061452.GV31174@grmbl.mre>
Date:	Wed, 21 Jan 2015 11:44:52 +0530
From:	Amit Shah <amit.shah@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	cornelia.huck@...ibm.com, Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v3 04/16] virtio/console: verify device has config space

On (Tue) 20 Jan 2015 [13:09:55], Michael S. Tsirkin wrote:
> On Tue, Jan 20, 2015 at 04:10:40PM +0530, Amit Shah wrote:
> > On (Wed) 14 Jan 2015 [19:27:35], Michael S. Tsirkin wrote:
> > > Some devices might not implement config space access
> > > (e.g. remoteproc used not to - before 3.9).
> > > virtio/console needs config space access so make it
> > > fail gracefully if not there.
> > 
> > Do we know any such devices?  Wondering what prompted this patch.  If
> > it's just theoretical, I'd rather let it be like this, and pull this
> > in when there's a device that doesn't have config space.
> 
> Yes, with virtio 1.0 config space can be in a separate BAR now.  If
> that's not enabled by BIOS (e.g. out of space), we won't have config
> space.

I'm still not sure whether we should pull in this patch before
actually seeing a failure.

You do have a dev_err which tells why the probe failed, so it's an
acceptable compromise I suppose.

> > Also, just the console functionality (i.e. F_MULTIPORT is unset) is
> > available w/o config space access.
> 
> Supporting this by gracefully disabling F_MULTIPORT
> would require getting this info from driver before
> features are finalized.
> Alternatively, check F_MULTIPORT and only fail if set?
> Let me know, I'll cook up a patch.

Yes, failing only if F_MULTIPORT is set is a better option (if we have
to fail).

> > In fact, getting this patch in
> > would mean remoteproc wouldn't even run in its pre-config days...
> 
> It seems to have get callback unconditionally now - or did I miss
> something?

What I meant was remoteproc doesn't depend on the config space, only
uses the console functionality.  If remoteproc devices didn't expose a
config space, this patch would cause it to lose its console
functionality for no apparent reason.


		Amit
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