[Qemu-devel] Re: Question about qemu firmware configuration (fw_cfg) device

[Gleb Natapov]

On Sat, Jul 17, 2010 at 10:50:59AM +0100, Richard W.M. Jones wrote:
> I'm trying to speed up the process of loading kernel and initrd.
> 
> I found that the main loop which loads these into qemu memory does it
> via executing in the guest:
> 
>   rep insb (%dx),%es:(%edi)
> 
> In other words, reading it byte-at-a-time from an emulated IO port.
> This is very slow[1] when your initrd is > 100MB like mine is.
> 
> Questions:
> 
> Is fw_cfg a purely qemu concept?  Does this BIOS firmware port
> 0x510-0x511 exist in real hardware?
> 
It is purely qemu concept.

> I understand from the git logs that fw_cfg was added because the old
> way was to load kernel & initrd into RAM directly, but this didn't
> work because SeaBIOS would clear the RAM, clobbering kernel & initrd.
> Could we change to loading these directly into RAM, and instead
> provide some indication to SeaBIOS so it doesn't clobber the RAM?  I'm
> quite prepared to do the work, just wondering if there's something
> else I'm not getting about this.
> 
> Rich.
> 
> [1] Several seconds of wallclock time, and according to gprof, the
> function 'fw_cfg_io_readb' accounts for > 50% of the time taken in
> qemu between qemu starting and us entering the Linux kernel.
> 
Several seconds is not too bad for 100M. Have you tested how much insl
improves this?

--
                        Gleb.