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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Sep 29, 2009 at 12:26:13PM -0500, Anthony Liguori wrote:
> Michael S. Tsirkin wrote:
>> 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.
>>   
>
> The host always exposes the config in two places.  config[0..235] in  
> bytes 20-255 in BAR0 and the full config space in BAR2 (or BAR1 + offset  
> if desired).  Old drivers will just ignore the new config location.
>
>> 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
>>   
>
> This is at best a band aid as a large number of small fields can result  
> in the same problem.

It would have to be very large :)

>> - For huge fields, provide a mailbox in i/o space where guest
>>   writes an offset and reads out a value
>>   
>
> Regards,
>
> Anthony Liguori