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 08:21:50PM +1030, Rusty Russell wrote:
>> "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
>
> OK so this means that you also need to track which bytes where written
> in order to know to skip byte 2.
> This is what I referred to. If instead we ask driver to specify
> offset/length explicitly device only needs to remember that.

I was assuming the implementation would keep two complete copies of the
config space: writes go to the scratch version, which gets copied to the
master version upon latch write.

But I do wonder if we should just skip this for now, since we don't
have any immediate need.

Cheers,
Rusty.