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
