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

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Message-ID: <20111106213849.GA14292@redhat.com>
Date:	Sun, 6 Nov 2011 23:38:49 +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 Sun, Nov 06, 2011 at 10:24:57PM +0200, Sasha Levin wrote:
> On Sun, 2011-11-06 at 09:30 +0200, Michael S. Tsirkin wrote:
> > 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.
> 
> They should be enabled, but heres the difference between the two
> approaches is that if it's cap it simply won't be there,

How can it not be there? They layout is specified by host,
not by guest.

> while in the
> other case it would just remain empty at some random offset of the
> struct.
> 
> -- 
> 
> Sasha.
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