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Re: [Qemu-devel] Hack integrating SeaBios / LinuxBoot option rom with QE
From: |
Alexander Graf |
Subject: |
Re: [Qemu-devel] Hack integrating SeaBios / LinuxBoot option rom with QEMU trace backends |
Date: |
Mon, 10 Oct 2011 21:01:52 +0200 |
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.
Alex
- [Qemu-devel] Hack integrating SeaBios / LinuxBoot option rom with QEMU trace backends, Daniel P. Berrange, 2011/10/10
- Re: [Qemu-devel] Hack integrating SeaBios / LinuxBoot option rom with QEMU trace backends, Anthony Liguori, 2011/10/10
- Re: [Qemu-devel] Hack integrating SeaBios / LinuxBoot option rom with QEMU trace backends,
Alexander Graf <=
- [Qemu-devel] Slow kernel/initrd loading via fw_cfg; Was Re: Hack integrating SeaBios / LinuxBoot option rom with QEMU trace backends, Daniel P. Berrange, 2011/10/11
- Re: [Qemu-devel] Slow kernel/initrd loading via fw_cfg; Was Re: Hack integrating SeaBios / LinuxBoot option rom with QEMU trace backends, Richard W.M. Jones, 2011/10/11
- Re: [Qemu-devel] Slow kernel/initrd loading via fw_cfg; Was Re: Hack integrating SeaBios / LinuxBoot option rom with QEMU trace backends, Avi Kivity, 2011/10/11
- Re: [Qemu-devel] Slow kernel/initrd loading via fw_cfg; Was Re: Hack integrating SeaBios / LinuxBoot option rom with QEMU trace backends, Alexander Graf, 2011/10/11
- Re: [Qemu-devel] Slow kernel/initrd loading via fw_cfg; Was Re: Hack integrating SeaBios / LinuxBoot option rom with QEMU trace backends, Avi Kivity, 2011/10/11
- Re: [Qemu-devel] Slow kernel/initrd loading via fw_cfg; Was Re: Hack integrating SeaBios / LinuxBoot option rom with QEMU trace backends, Alexander Graf, 2011/10/11
- Re: [Qemu-devel] Slow kernel/initrd loading via fw_cfg; Was Re: Hack integrating SeaBios / LinuxBoot option rom with QEMU trace backends, Avi Kivity, 2011/10/11
- Re: [Qemu-devel] Slow kernel/initrd loading via fw_cfg; Was Re: Hack integrating SeaBios / LinuxBoot option rom with QEMU trace backends, Gleb Natapov, 2011/10/11
- Re: [Qemu-devel] Slow kernel/initrd loading via fw_cfg; Was Re: Hack integrating SeaBios / LinuxBoot option rom with QEMU trace backends, Avi Kivity, 2011/10/11
- Re: [Qemu-devel] Slow kernel/initrd loading via fw_cfg; Was Re: Hack integrating SeaBios / LinuxBoot option rom with QEMU trace backends, Gleb Natapov, 2011/10/11