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

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Date:	Sun, 6 Nov 2011 09:30:07 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	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 Fri, Nov 04, 2011 at 04:53:05PM +0200, Sasha Levin wrote:
> > > > > As you said, the PCI cap list was introduced both to save space (which
> > > > > is not the motivation here), and because it's a very efficient
> > > > 
> > > > It's actually pretty inefficient - there's an overhead of 3 bytes for
> > > > each vendor specific option.
> > > 
> > > It's efficient because while you pay a small price for each optional
> > > option it also means that that option is optional and won't clutter the
> > > config space if it's not really in use.
> > 
> > I guess my assumption is that most options will be in use,
> > not discarded dead-ends.
> 
> I don't know about that. 64 bit features would be pretty rare for now -
> and I don't think that setting the alignment will be also enabled by
> default.

Setting the alignment might not be *used* by default but
I think it must be enabled by default to allow bios access.

> I think that we're looking at it differently because I assume that any
> feature we add at this point would be optional and used only in specific
> scenarios, while you think that everything added will be used most of
> the time.

Options must often be present even if not used. For example, as device
has no way to know whether a guest will want to program alignment, it
has to make that option available.

-- 
MST
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