Re: [Qemu-devel] [RFC 2/7] fw_cfg dma interface

[Kevin O'Connor <kevin@koconnor.net>]

On Wed, Jul 22, 2015 at 10:31:12AM +0200, Marc Marí wrote:
> On Wed, 22 Jul 2015 00:24:34 -0400
> "Kevin O'Connor" <kevin@koconnor.net> wrote:
> > On Tue, Jul 21, 2015 at 06:03:41PM +0200, Marc Marí wrote:
> > > From: Gerd Hoffmann <kraxel@redhat.com>
> > > 
> > > First draft of a fw_cfg dma interface.  Designed as add-on to the
> > > extisting fw_cfg interface, i.e. there is no select register.  There
> > > are four 32bit registers:  Target address (low and high bits),
> > > transfer length, control register.
> > 
> > If I read this interface correctly, a guest will have at least six
> > faults to complete a typical fw_cfg dma transfer (select, target low,
> > target high, transfer length, control register write, control register
> > read).  I wonder if using a DMA transfer descriptor might be more
> > efficient.
[...]
> That is probably faster and more flexible. I think it's a good step
> forward. But, on the other side, is this a too big break of what is
> actually implemented in MMIO?
> 
> At the end we will have MMIO version, with a select and a data
> register, and DMA version, with a 64 bit address register. How can we
> differentiate between versions? Laszlo talks about the ID, but this is
> already in one of the fields (so you need to know the protocol to get
> the protocol). How could we do this transition more seamless?

Well, one way would be to place a 64bit MMIO (or IO) register at a
magic address.  The firmware could then read the magic address and
check it against a signature (eg, "QEMU CFG").  If the signature
matches then it would know it could use the DMA version of the fw_cfg
interface (by issuing writes, instead of reads, to the magic address).
The firmware would then not have to use the older select/data fw_cfg
interface at all.

Another possibility would be to place the new fw_cfg dma register
address into a named fw_cfg "file" (eg, "fw_cfg_dma").  The firmware
could then use the existing select/data fw_cfg interface to check if
the new dma interface is available by scanning for that "fw_cfg_dma"
file.  This has the advantage of not requiring a new "magic address",
but has the disadvantage of a more complex probe.

Just a thought,
-Kevin