From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [RFC 7/11] virtio_pci: new, capability-aware driver.
Date: Mon, 19 Dec 2011 11:13:26 +0200
Message-ID: <20111219091324.GA19535@redhat.com>
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To: Rusty Russell <rusty@rustcorp.com.au>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>, Sasha Levin <levinsasha928@gmail.com>, Pawel Moll <pawel.moll@arm.com>, virtualization <virtualization@lists.linux-foundation.org>
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On Mon, Dec 19, 2011 at 04:36:38PM +1030, Rusty Russell wrote:
> On Sun, 18 Dec 2011 12:18:32 +0200, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > On Fri, Dec 16, 2011 at 12:20:08PM +1030, Rusty Russell wrote:
> > > Perhaps a new feature VIRTIO_F_UNSTABLE?  Which (unlike other features)
> > > appears and vanishes around config writes by either side?  Kind of a
> > > hack though...
> > 
> > Not sure how this can work in such a setup: when would guest
> > check this bit to avoid races?
> > A separate registers also seems nicer than a flag.
> > 
> > Some other possible design choices:
> > - a flag to signal config accesses in progress by guest
> >   host would need to buffer changes and apply them in one go
> >   when flag is cleared
> > - a register to make host get/set config in guest memory
> > - use a control vq for all devices
> 
> - seqlock-style generation count register(s)?
>   Has the advantage of
>   being a noop if things never change.

The counter can be in guest memory, right? So we don't pay extra exits.

> - continue to ignore it ;)

Since you decided on a config layout redesign it seems a good time to
fix architectural problems ...

> And yes, it's a more general problem than virtio_pci...
> 
> Cheers,
> Rusty.


-- 
MST