linux-kernel - Re: QEMU fw_cfg DMA interface

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Date:	Thu, 6 Aug 2015 17:53:26 +0200
From:	Marc Marí <markmb@...hat.com>
To:	"Kevin O'Connor" <kevin@...onnor.net>
Cc:	Stefan Hajnoczi <stefanha@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	qemu-devel <qemu-devel@...gnu.org>, seabios@...bios.org,
	Drew <drjones@...hat.com>, Gerd Hoffmann <kraxel@...hat.com>,
	Laszlo <lersek@...hat.com>
Subject: Re: QEMU fw_cfg DMA interface

On Thu, 6 Aug 2015 11:30:43 -0400
"Kevin O'Connor" <kevin@...onnor.net> wrote:

> On Thu, Aug 06, 2015 at 02:37:45PM +0200, Marc Marí wrote:
> > On Thu, 6 Aug 2015 13:27:16 +0100
> > Stefan Hajnoczi <stefanha@...il.com> wrote:
> > 
> > > On Thu, Aug 6, 2015 at 12:00 PM, Marc Marí <markmb@...hat.com>
> > > wrote:
> > > > When running a Linux guest on top of QEMU, using the -kernel
> > > > options, this is the timing improvement for x86:
> > > >
> > > > QEMU commit 2be4f242b50a8 and SeaBIOS commit 908a58c1d5ff
> > > > QEMU startup time: .078
> > > > BIOS startup time: .060
> > > > Kernel setup time: .578
> > > > Total time: .716
> > > >
> > > > QEMU with this patch series and SeaBIOS with this patch series
> > > > QEMU startup time: .080
> > > > BIOS startup time: .039
> > > > Kernel setup time: .002
> > > > Total time: .121
> > > 
> > > Impressive results!
> > > 
> > > Is this a fully-featured QEMU build or did you disable things?
> > > 
> > > Is this the default SeaBIOS build or did you disable things?
> > > 
> > 
> > This is the default QEMU configuration I get for my system. It's
> > not a fully-featured QEMU, but it has a lot of things enabled.
> > SeaBIOS has a default configuration (with debugging disabled).
> 
> Thanks!
> 
> What qemu command-line did you use during testing?  Also, do you have
> a quick primer on how to use the kvm trace stuff to obtain timings?
> 

The command line I used is:

x86_64-softmmu/qemu-system-x86_64 --enable-kvm \
    -kernel /boot/vmlinuz-4.0.7-300.fc22.x86_64 \
    -L pc-bios/optionrom/ \
    -bios roms/seabios/out/bios.bin -nographic

And I used perf (and two out instructions in the BIOS) to measure the
times:

perf record -a -e kvm:\* -e sched:sched_process_exec

And searching for sched:sched_process_exec, kvm:kvm_entry, pio_write at
0xf5 and pio_write at 0xf4. Out at 0xf5 is the one in "do_boot"
function, and out at 0xf4 is the one just before the jump to the Linux
kernel.

I attach the script I've been using. It can be improved, but it works.
It can be run like this:

sudo ../../results/run_test.sh x86_64-softmmu/qemu-system-x86_64 \
    --enable-kvm -kernel /boot/vmlinuz-4.0.8-300.fc22.x86_64 \
    -L pc-bios/optionrom/ \
    -bios roms/seabios/out/bios.bin -nographic

Thanks
Marc

Download attachment "run_test.sh" of type "application/x-shellscript" (1401 bytes)