Re: [Qemu-devel] RFC: vfio / device assignment -- layout of device fd files

[Scott Wood]

On 08/29/2011 02:04 PM, Anthony Liguori wrote:
> On 08/29/2011 11:51 AM, Yoder Stuart-B08248 wrote:
>> Instead of config space fixed at 0xf, we would propose
>> a header and multiple 'device info' records at offset 0x0 that would
>> encode everything that user space needs to know about
>> the device:
> 
> Why not just use an ioctl with a proper struct?

This is more extensible than a struct -- both in features, and in the
number of each type of resource that you can have, length of strings you
can have, etc.

> The config space is weird for PCI access because it's mirroring a well
> known binary blob.  It's not something to replicate if you're inventing
> something new.

There's no intent to replicate config space in general -- config space
is provided as-is.  There's little overlap between config space and the
extra information provided.  Length can be had from config space, but
only by modifying it.  Physical address sort-of overlaps, though bus
addresess could be different from CPU physical addresses[1].  In both
cases, it'd be nice to stay consistent with device-tree regions.

"BAR type" is overlap, but doesn't seem too unreasonable to me.

-Scott

[1] The user is probably less likely to care about the physical address
at all in the PCI case, but consistency is nice.