Re: [Qemu-devel] [PATCH v3 4/5] Enable fw_cfg DMA interface for ARM

[Gabriel L. Somlo]

On Mon, Oct 26, 2015 at 10:48:08AM +0000, Stefan Hajnoczi wrote:
> On Thu, Oct 22, 2015 at 05:22:16PM -0400, Gabriel L. Somlo wrote:
> > I was re-reading the documentation for fw_cfg_add_file_callback(),
> > and noticed that non-dma read operations check for the presence
> > of a callback (and call it if present) for *every* *single* *byte*,
> > even on 64-bit MMIO reads. That's also what the documentation says
> > (in docs/specs/fw_cfg.txt, being moved into fw_cfg.h as per
> >  http://lists.nongnu.org/archive/html/qemu-devel/2015-10/msg05315.html).
> > 
> > During DMA reads, however, the callback is only checked once before
> > each chunk, effectively once per DMA read operation.
> > 
> > Now, typical callbacks I found throughout the qemu source tend to return
> > immediately except for the first time they're invoked, but I wonder if
> > skipping over all those extra "do I have a callback, if so call it,
> > mostly so it can return without doing anything" per-byte operations
> > account in some significant part for the dramatically faster transfers?
> > 
> > Not sure how I'd test for that -- besides my not having anything
> > resembling a viable ARM setup, I'm not sure if limiting the callbacks
> > to only be invoked if (s->cur_offset == 0) would make sense, just as a
> > test ?
> 
> I think Marc came to the conclusion that it's safe and therefore made
> that optimization for DMA.
> 
> The same can be done for PIO.

OK, so at the risk of over-reaching here, would it make sense to
rewrite the fw_cfg spec to say "If present, a callback will be
executed *once* before each time a blob is read" ?

My hypothesis (which I guess I'm volunteering to verify, unless we
end up rejecting this immediately as a bad idea, for some reason that
I have missed), is that current functionality wouldn't change, given
the way existing callbacks work right now, and that we could run the
callback each time a blob is *selected*, rather than hooking into the
(dma/mmio/pio) read methods.

Thanks,
--Gabriel