Re: [Qemu-devel] Using PCI config space to indicate config location

[Rusty Russell]

"Michael S. Tsirkin" <address@hidden> writes:
> On Fri, Oct 12, 2012 at 08:59:36AM +1030, Rusty Russell wrote:
>> >> For writes, the standard seems to be a commit latch.  We could abuse the
>> >> generation count for this: the driver writes to it to commit config
>> >> changes.
>> >
>> > I think this will work. There are a couple of things that bother me:
>> >
>> > This assumes read accesses have no side effects, and these are sometimes 
>> > handy.
>> > Also the semantics for write aren't very clear to me.
>> > I guess device must buffer data until generation count write?
>> > This assumes the device has a buffer to store writes,
>> > and it must track each byte written. I kind of dislike this
>> > tracking of accessed bytes. Also, device would need to resolve conflicts
>> > if any in some device specific way.
>> 
>> It should be trivial to implement: you keep a scratch copy of the config
>> space, and copy it to the master copy when they hit the latch.
>> 
>> Implementation of this will show whether I've missed anything here, I
>> think.
>
> What I refer to: what happens if driver does:
> - write offset 1
> - write offset 3
> - hit commit latch

- nothing
- nothing
- effect of offset 1 and offset 3 writes

Now, since there's nothing published by the *driver* at the moment
which can't be trivially atomically written, this scheme is overkill
(sure, it means you could do a byte-at-a-time write to some 4-byte
field, but why?).

But perhaps it's overkill: no other bus has this feature, so we'd need a
feature bit for them anyway in future if we create a device which needs
such atomicity.

Cheers,
Rusty.