linux-kernel - Re: virtio-pci new configuration proposal

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Date:	Fri, 04 Nov 2011 20:14:43 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>,
	virtualization <virtualization@...ts.linux-foundation.org>,
	Anthony Liguori <anthony@...emonkey.ws>
Subject: Re: virtio-pci new configuration proposal

On Thu, 03 Nov 2011 10:33:23 +0200, Sasha Levin <levinsasha928@...il.com> wrote:
> On Thu, 2011-11-03 at 12:28 +1030, Rusty Russell wrote: 
> > 2) I don't think we want to turn the device-specific config into a
> >    linked list.  We haven't needed variable-length config (yet!), and
> >    it's (slightly) more complex.  That's also the part of the spec which
> >    is shared with non-PCI virtio implementations.
> 
> Variable length config wasn't used yet because space in the device
> specific space was reserved for a feature even if that feature wasn't
> used.
> 
> For example, the MAC feature reserved 6 bytes in the config space for
> the MAC even if VIRTIO_NET_F_MAC wasn't enabled. Here we can just avoid
> having it pollute the config space until it's enabled.

Exactly.  But we haven't had a problem so far; but we don't put
arbitrarily large fields in there.

> I don't think it'll have any impact on non-PCI implementations since the
> "pointers" are simply offsets from the beginning of the config space,
> and are not PCI specific in any way.

But the drivers currently just use offsetof() to access it, eg:

	if (virtio_config_val_len(vdev, VIRTIO_NET_F_MAC,
				  offsetof(struct virtio_net_config, mac),
				  dev->dev_addr, dev->addr_len) < 0)

That would have to change, and that means a change for drivers and for
the non-PCI implementations.

Hence I think this is a step too far.

> > 3) If we're changing the queue layout, it's a chance to fix a
> >    longstanding bug: let the guest notify the host of preferred
> >    queue size and alignment.
> 
> Yup, we can do that.

The seabios guys will definitely thank you!

Cheers,
Rusty.
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