[SeaBIOS] [Qemu-devel] Hack integrating SeaBios / LinuxBoot option rom with QEMU trace backends

Alexander Graf agraf at suse.de
Mon Oct 10 21:01:52 CEST 2011

On 10.10.2011, at 20:53, Anthony Liguori wrote:

> On 10/10/2011 12:08 PM, Daniel P. Berrange wrote:
>> I've been investigating where time disappears to when booting Linux guests.
>> Initially I enabled DEBUG_BIOS in QEMU's hw/pc.c, and then hacked it so
>> that it could print a timestamp before each new line of debug output. The
>> problem with that is that it slowed down startup, so the timings I was
>> examining all changed.
>> What I really wanted was to use QEMU's trace infrastructure with a simple
>> SystemTAP script. This is easy enough in the QEMU layer, but I also need
>> to see where time goes to inside the various BIOS functions, and the
>> options ROMs such as LinuxBoot. So I came up with a small hack to insert
>> "probes" into SeaBios and LinuxBoot, which trigger a special IO port
>> (0x404), which then cause QEMU to emit a trace event.
>> The implementation is really very crude and does not allow any arguments
>> to be passed each probes, but since all I care about is timing information,
>> it is good enough for my needs.
>> I'm not really expecting these patches to be merged into QEMU/SeaBios
>> since they're just a crude hack&  I don't have time to write something
>> better. I figure they might be useful for someone else though...
>> With the attached patches applied to QEMU and SeaBios, the attached
>> systemtap script can be used to debug timings in QEMU startup.
>> For example, one execution of QEMU produced the following log:
>>   $ stap qemu-timing.stp
>>   0.000 Start
>>   0.036 Run
>>   0.038 BIOS post
>>   0.180 BIOS int 19
>>   0.181 BIOS boot OS
>>   0.181 LinuxBoot copy kernel
>>   1.371 LinuxBoot copy initrd
> Yeah, there was a thread a bit ago about the performance of the interface to read the kernel/initrd.  I think at it was using single byte access instructions and there were patches to use string accessors instead?  I can't remember where that threaded ended up.

IIRC we're already using string accessors, but are still slow. Richard had a nice patch cooked up to basically have the fw_cfg interface be able to DMA its data to the guest. I like the idea. Avi did not.

And yes, bad -kernel performance does hurt in some workloads. A lot.


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