Re: [Qemu-devel] Re: [PATCH] qemu: make virtio-blk PCI compliant by default

[john cooper <john.cooper@third-harmonic.com>]

Anthony Liguori wrote:
> john cooper wrote:
>> Avi Kivity wrote:
>>  
>>> On 09/22/2009 05:21 PM, john cooper wrote:
>>>    
>>>>> Can we just read this page as a virtqueue command instead of having it
>>>>> mapped permanently?
>>>>>              
>>>> Probably although I hadn't looked specifically
>>>> at doing so.   Mapping the data via an unused
>>>> pci bar is pretty trivial and seemed minimally
>>>> intrusive to the existing driver.
>>>>          
>>> We'll run out of bars if we expend them like that.
>>>     
>>
>> Agreed.  However my motivation here was to use a
>> single additional bar specifically to compensate
>> for the PCI spec imposed 256 byte size limitation
>> of the config space mapping.  As we're defining the
>> content/size of this area, future use to accommodate
>> additional data should be unrestricted.
>>   
> 
> Why expose the whole ATAPI page instead of just the serial number?

Exposing the s/n alone was what I'd done originally
which in retrospect was well contained within the
pci config area -- even if squandering the somewhat
limited space in that area.

However exposing it as an identify page allows use
of the existing HDIO_GET_IDENTITY ioctl (and existing
use of that interface, eg: "hdparm -i") as a means to
extract the s/n along with potentially useful related
data in the page.

Creation of the identify structure is within qemu
as that is where the information exists.  And this
allows the virtio_blk driver to treat the data as
opaque, simply exporting it uninterpreted via a copyout.
So the only real issue remaining is how to best saw
a hole between qemu and the guest virtio_blk driver
to transfer the identity data.

> I think the proper solution is to move the config to a separate bar 
> that's MMIO instead of PIO.  config access is never performance 
> sensitive and an MMIO bar has less restrictions on size.

That's exactly what I've done.  I'll clean up the
patch and post it here so we can have a more concrete
discussion.

-john 


-- 
john.cooper@third-harmonic.com