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

[Anthony Liguori <anthony@codemonkey.ws>]

On 01/10/2012 06:25 PM, Rusty Russell wrote:
> On Tue, 10 Jan 2012 19:03:36 +0200, "Michael S. Tsirkin"<mst@redhat.com>  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

I think the more important thing to do is require accesses to integers in the 
config space to always be aligned and to use the appropriate accessor. 
Non-integer fields should be restricted to byte access.

That limits config space entries to 32-bit but also means that there is no need 
for a generation counter.  It's also easier to deal with endian conversion that way.

But it means the backend code ends up being much simpler to write (because it 
behaves more like a normal PCI device).

If we're already making the change, the endianness ought to be a feature bit.

> We should also change the ring (to a single ring, I think).

Ack.

> 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.

Ack.  Long live virtio2! :-)

Regards,

Anthony Liguori

>
> Thoughts?
> Rusty.