Re: [Qemu-devel] RFC: Proposed vfio IGD assignment fw_cfg ABI

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 13 May 2016 10:21:00 +0200
Gerd Hoffmann <kraxel@redhat.com> wrote:

> > #1: "etc/igd-opregion"
> > 
> > the IGD OpRegion is an area of memory which contains among other
> > things, the Video BIOS Table which is integral in allowing an assigned
> > IGD to configure and make use of the physical display outputs of the
> > system.  "etc/igd-opregion" is an opaque fw_cfg file which the BIOS
> > will use to allocate an appropriately sized reserved memory region,
> > copy the contents of the fw_cfg file into the allocated memory region,
> > and write the base address of the allocated memory region to the dword
> > registers at 0xFC in PCI config space on the IGD device itself.  The
> > BIOS will look for this fw_cfg file any time a PCI class VGA device is
> > found with Intel vendor ID.  Multiple IGD devices per VM, such as might
> > potentially be possible with Intel vGPU, is not within the scope of
> > this proposal.  The expected size of this fw_cfg file is on the order
> > of a few pages, 8KB is typical.  
> 
> Looks good to me.
> 
> > #2: "etc/igd-bdsm"  
> 
> > possibility of multiple BDSM per VM.  The expected size of this fw_cfg
> > file is from 1MB to multiple hundreds of MB with user specified stolen
> > video memory.  
> 
> Having a big fw_cfg file without using the content looks a bit odd to
> me.  I'd suggest to create a fw_cfg file with the size in it instead.
> Usual way to do this is to stick a 64bit value in little endian byte
> order into fw_cfg, then use romfile_loadint() in seabios to read it.

I think following this approach I'd rename the fw_cfg file to
"etc/igd-bdsm-size", otherwise looks reasonable, I'll make the change.

> > I'd appreciate any comments on these entries and I'd be happy to
> > describe them further.  Perhaps we should create a docs/api/
> > directory with these sorts of descriptions where we define how a
> > given API is intended to work.  
> 
> There is docs/specs/ already where this would fit, and it even has a
> fw_cfg.txt file.  It might make sense to have a separate igd-assign.txt
> though.

Yeah, even beyond these fw_cfg options there's probably enough oddity
with IGD assignment to have it's own docs file.  I'll cook something up
for the next posting.  Thanks for the review and comments.

Alex