Re: [Qemu-devel] [PATCH v2] PAM: make PAM emulation closer to documentation

[Kevin O'Connor <kevin@koconnor.net>]

On Mon, Jul 20, 2015 at 01:53:13PM +0300, Efimov Vasily wrote:
> This patch improves PAM emulation.
> 
> PAM defines 4 memory access redirection modes. In mode 1 reads are directed to
> RAM and writes are directed to PCI. In mode 2 it is contrary. In mode 0 all
> access is directed to PCI. In mode 3 it is directed to RAM. Currently all modes
> are emulated using aliases. It is good for modes 0 and 3 but modes 1 and 2
> require more complicated logic. Present API has not needed region type.

Hi,

Is the motivation of this patch to improve the correctness of the
hardware emulation or is there some feature that the current QEMU
implementation prevents?

There has been some interest recently in making SeaBIOS boot faster on
QEMU and the shadow ram fixups in SeaBIOS have been shown to consume a
notable amount of bootup time (~12ms on my machine).

Will changing the PAM support improve overall boot times?  I wonder if
going from "mode 0" -> "mode 2" -> "mode 3" would be more expensive
than the current SeaBIOS "mode 0" -> "mode 3" mechanism.

Also note that SeaBIOS would still need to copy from the high memory
location even if the PAM registers were fully implemented as SeaBIOS
often exceeds 128K - the "mode 2" overwrite oneself trick would only
work for the last 128K of the rom.

Thanks,
-Kevin