Re: [Qemu-devel] [PATCH v2] arm: add fw_cfg to "virt" board

[Laszlo Ersek <lersek@redhat.com>]

On 12/09/14 10:31, Gerd Hoffmann wrote:
>   Hi,
> 
>> So... after playing with this thing for some time, it's become clear
>> that "MMIO traps" are painfully slow on the aarch64 platform we've been
>> working on (using KVM).
> 
> So, as we don't have compatibility requirements, and we also can't play
> tricks like using x86 string instructions:  How about a completely
> different, dma-style interface for fw_cfg access?
> 
> One register for the (physical) target address.
> One register for the transfer size.
> One register for the fw_cfg entry.
> Possibly one register for the fw_cfg offset (not really needed, but
> avoids the need for side effects such as writing fw_cfg entry register
> clearing the offset).
> One register to kick the transfer.

Again, this was the idea that Rich had in 2010 (see the links in the
discussion thus far). It was rejected back then (which is why I didn't
even try to resurrect it now), and Peter has now asked if anything has
changed that would make that approach more acceptable now.

"Avi and Gleb not hanging around so they can't block the approach now"
might or might not be such a change; I don't know. But, I tend to trust
their opinion, even if dates back to 2010 in this case.

Anyway I'm certainly not opposed to performance, so if someone can
thoroughly refute everything they said in that thread, go ahead.

Thanks!
Laszlo