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

[Gerd Hoffmann]

  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.

cheers,
  Gerd