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

[Stefan Hajnoczi]

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.

Stefan