Re: [PATCH RFC] virtio-pci: new config layout: using memory BAR

[Gerd Hoffmann <kraxel@redhat.com>]

On 06/06/13 08:34, Gleb Natapov wrote:
> On Wed, Jun 05, 2013 at 07:41:17PM -0500, Anthony Liguori wrote:
>>
>> Oh, you mean in real mode.
>>
>> SeaBIOS runs the virtio code in 32-bit mode with a flat memory layout.
>> There are loads of ASSERT32FLAT()s in the code to make sure of this.
>>
> Well, not exactly. Initialization is done in 32bit, but disk
> reads/writes are done in 16bit mode since it should work from int13
> interrupt handler.

Exactly.  It's "only" the initialization code which has ASSERt32FLAT()
all over the place.  Which actually is the majority of the code in most
cases as all the hardware detection and initialization code is there.
But kicking I/O requests must work from 16bit mode too.

> The only way I know to access MMIO bars from 16 bit
> is to use SMM which we do not have in KVM.

For seabios itself this isn't a big issue, see pci_{readl,writel} in
src/pci.c.  When called in 16bit mode it goes into 32bit mode
temporarily, just for accessing the mmio register.  ahci driver uses it,
xhci driver (wip atm) will use that too, and virtio-{blk,scsi} drivers
in seabios can do the same.

But as hpa mentioned it will be more tricky for option roms (aka
virtio-net).

cheers,
  Gerd