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

["Richard W.M. Jones" <rjones@redhat.com>]

On Sun, Jul 18, 2010 at 07:26:57PM +0200, Alexander Graf wrote:
> On 17.07.2010, at 11:53, Richard W.M. Jones wrote:
> > On Sat, Jul 17, 2010 at 10:50:59AM +0100, Richard W.M. Jones wrote:
> >> 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.
> > 
> > Or thinking around the subject:
> > 
> > Change fw_cfg so that you send a command + a physical address, and
> > fw_cfg memcpy's the kernel / initrd / etc to that physical address.
> > Then linuxboot.bin doesn't have to do the manual copying.
> > 
> > Or just change linuxboot.bin so it does 32 bit inl instructions, which
> > might at least be a bit faster ...
> 
> I don't see why it would be slow. ins should be emulated using coalesced mmio, no?

It knocks 1 second off libguestfs boot times on my faster 64 bit
desktop machine, and 2 seconds off with my old 32 bit laptop.
(Roughly 15% faster in both cases)

The 64 bit machine times are:

Without my patch:

real	0m7.581s
user	0m4.730s
sys	0m2.124s

With my patch:

real	0m6.579s
user	0m3.614s
sys	0m1.941s

If you want to reproduce this (you'll need a recent Fedora machine)
you can do:

  $ cat qemu-wrapper 
  #!/bin/sh -
  qemudir=/home/rjones/d/qemu
  exec $qemudir/x86_64-softmmu/qemu-system-x86_64 -L $qemudir/pc-bios "$@"

  $ export LIBGUESTFS_QEMU=/home/rjones/d/qemu/qemu-wrapper
  $ time guestfish --ro -a /dev/null run

Obviously I'm running that several times over, discarding the first
few runs, because I'm only interested in the "hot cache" case.

By the way, even if you reject the patch as a whole, part 1/2 of the
patch is just an obvious bug fix, and I think should be applied
anyway.

http://lists.gnu.org/archive/html/qemu-devel/2010-07/threads.html#00967

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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