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

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Message-ID: <20150121064409.GA13320@redhat.com>
Date:	Wed, 21 Jan 2015 08:44:09 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Amit Shah <amit.shah@...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 Wed, Jan 21, 2015 at 11:44:52AM +0530, Amit Shah wrote:
> 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).

OK, that's easy I think - will send a patch on top.

> > > 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

Better than crashing on jump to NULL?

-- 
MST
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