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[Qemu-devel] Re: [PATCH 0/2] fix virtio_blk serial pci config breakage

From: Anthony Liguori
Subject: [Qemu-devel] Re: [PATCH 0/2] fix virtio_blk serial pci config breakage
Date: Tue, 29 Sep 2009 12:26:13 -0500
User-agent: Thunderbird (X11/20090825)

Michael S. Tsirkin wrote:
On Tue, Sep 29, 2009 at 09:14:56AM -0500, Anthony Liguori wrote:
Michael S. Tsirkin wrote:
I don't think it's such a good idea.  We want to keep exposing most of
config space in i/o bar because of backwards compatibility.  And we want
to keep datapath operations such as vq kicks, in i/o space.
We can do feature negotiation in virtio-pci. That lets us continue exposing the first 246 bytes of the config space in PIO and guests can negotiate a second bar for access to larger config spaces.


Anthony Liguori

guests can't negotiate a bar.

The host always exposes the config in two places. config[0..235] in bytes 20-255 in BAR0 and the full config space in BAR2 (or BAR1 + offset if desired). Old drivers will just ignore the new config location.

What I think we want to do
- Use i/o for small fields (2-4 bytes) as we did previously
  we won't run out anytime soon
- For large fields, put them in memory BAR1 and report the
  offset in i/o space

This is at best a band aid as a large number of small fields can result in the same problem.

- For huge fields, provide a mailbox in i/o space where guest
  writes an offset and reads out a value


Anthony Liguori

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