[Qemu-devel] Re: [PATCH 0/2] fix virtio_blk serial pci config breakage

[Michael S. Tsirkin]

On Tue, Sep 29, 2009 at 09:14:56AM -0500, Anthony Liguori wrote:
> Michael S. Tsirkin wrote:
>> I don't think it's such a good idea.  We want to keep exposing most of
>> config space in i/o bar because of backwards compatibility.  And we want
>> to keep datapath operations such as vq kicks, in i/o space.
>>   
>
> We can do feature negotiation in virtio-pci.  That lets us continue  
> exposing the first 246 bytes of the config space in PIO and guests can  
> negotiate a second bar for access to larger config spaces.
>
> Regards,
>
> Anthony Liguori

guests can't negotiate a bar.

What I think we want to do
- Use i/o for small fields (2-4 bytes) as we did previously
  we won't run out anytime soon
- For large fields, put them in memory BAR1 and report the
  offset in i/o space
- For huge fields, provide a mailbox in i/o space where guest
  writes an offset and reads out a value

-- 
MST