Re: Using PCI config space to indicate config location

[Rusty Russell <rusty@rustcorp.com.au>]

"Michael S. Tsirkin" <mst@redhat.com> writes:
> On Tue, Oct 16, 2012 at 11:45:41PM +1030, Rusty Russell wrote:
>> "Michael S. Tsirkin" <mst@redhat.com> writes:
>> 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.
>
> Yes but config space has some host modifiable registers too.
> So host needs to be careful to avoid overwriting these.
>
> If accesses have side effects that of course breaks too ...

Yes.

>
>> But I do wonder if we should just skip this for now, since we don't
>> have any immediate need.
>> 
>> Cheers,
>> Rusty.
>
> MAC setting from guest needs this right now, no?

Ah, I missed that in my table:

    Driver          Config       Device changes    Driver writes... after init?
    net             Y            Y                 N                N
    block           Y            Y                 Y                Y
    console         Y            Y                 N                N
    rng             N            N                 N                N
    balloon         Y            Y                 Y                Y
    scsi            Y            N                 Y                N
    9p              Y            N                 N                N

First line should be:

    net             Y            Y                 Y                N

So we could add a new cvq command (eg. #define VIRTIO_NET_CTRL_MAC_SET
1) and a VIRTIO_NET_F_CVQ_MAC_SET feature.  Or go ahead with the
latching scheme (which doesn't really help other busses).

Cheers,
Rusty.