Re: [RFC 7/11] virtio_pci: new, capability-aware driver.

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On Tue, 10 Jan 2012 19:03:36 +0200, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:
> On Wed, Dec 21, 2011 at 11:03:25AM +1030, Rusty Russell wrote:
> > Yes.  The idea that we can alter fields in the device-specific config
> > area is flawed.  There may be cases where it doesn't matter, but as an
> > idea it was holed to begin with.
> > 
> > We can reduce probability by doing a double read to check, but there are
> > still cases where it will fail.
> Okay - want me to propose an interface for that?

Had a brief chat with BenH (CC'd).

I think we should deprecate writing to the config space.  Only balloon
does it AFAICT, and I can't quite figure out *why* it has an 'active'
field.  This solves half the problem, of sync guest writes.  For the
other half, I suggest a generation counter; odd means inconsistent.  The
guest can poll.

BenH also convinced me we should finally make the config space LE if
we're going to change things.  Since PCI is the most common transport,
guest-endian confuses people.  And it sucks for really weird machines.

We should also change the ring (to a single ring, I think).  Descriptors
to 24 bytes long (8 byte cookie, 8 byte addr, 4 byte len, 4 byte flags).
We might be able to squeeze it into 20 bytes but that means packing.  We
should support inline, chained or indirect.  Let the other side ack by
setting flag, cookie and len (if written).

Moreover, I think we should make all these changes at once (at least, in
the spec).  That makes it a big change, and it'll take longer to
develop, but makes it easy in the long run to differentiate legacy and
modern virtio.

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