On 19/08/14 16:21, Jakub Jermar wrote:
Hi Jakub,
On 17.8.2014 18:16, Mark Cave-Ayland wrote:
Another issue I've found with my OpenBIOS tests is that HelenOS PPC
appears to be quite sluggish under the latest qemu-system-ppc. I've
attempted a few bisections but it's quite tricky to tie the regression
down to one particular commit.
The slowness starts to appear in a minor form with the 1.6 timer rewrite
and seems to have become worse with newer QEMU releases. There have been
various changes to the OpenBIOS frequency/tb-frequency properties in
order to support OS X PPC in this timeframe so I'm wondering if somehow
HelenOS is exposing a bug in the qemu-system-ppc timer emulation?
In order to reproduce, grab a QEMU 2.1 release tarball, compile and
install and then launch like this:
qemu-system-ppc -cdrom HelenOS-0.5.0-ppc32.iso -m 512 -boot d
As soon as the main kernel executes you can see the delay in writing
characters to the VGA console.
I also noticed that as of QEMU 2.0, HelenOS/ppc32 under QEMU became
quite slow.
Now, I am not a ppc32 expert either, but I suspect that the problem is
related to the way HelenOS programs the decrementer exception. It always
does so using the instruction below:
mtdec reg ; reg = 1000
So we always use constant 1000, regardless of what the real frequency
property is. I guess this is a constant which used to work reasonably
well under PearPC back then and has stayed ever since. *Martin*, correct
me if I am wrong, please. Btw, was the actual frequency increased? That
could explain why HelenOS/ppc32 became slower - with more exceptions
you'd have bigger overhead.
Ah possibly that could be it. The file history with the relevant changes
in QEMU for -M g3beige (the default) can found at
http://git.qemu.org/?p=qemu.git;a=history;f=hw/ppc/mac_oldworld.c;h=cd9bdbc53e92f423a329ecce7de1df9244a5eb0b;hb=0e4a77370594c91dd126f9872893ed473374cc72.
In particular you probably want to look at these commits:
http://git.qemu.org/?p=qemu.git;a=commitdiff;h=a1014f25ef54bbbdaf571873a1254f85d6dcf132
http://git.qemu.org/?p=qemu.git;a=commitdiff;h=536d8cda4adef3d202a800688ed5f63dae1f0f88
http://git.qemu.org/?p=qemu.git;a=commitdiff;h=9d1c128341df7a303571f172d986291b4f3ed9ee
With the latest QEMU tree, all of the timebase frequency, clock
frequency and bus frequency are generated by QEMU and then passed to
OpenBIOS which creates the relevant device tree properties for the CPU node:
build@kentang:~/rel-qemu-git/bin$ ./qemu-system-ppc -nographic -prom-env
'auto-boot?=false'
>> =============================================================
>> OpenBIOS 1.1 [Jun 20 2014 22:58]
>> Configuration device id QEMU version 1 machine id 2
>> CPUs: 1
>> Memory: 128M
>> UUID: 00000000-0000-0000-0000-000000000000
>> CPU type PowerPC,750
milliseconds isn't unique.
Welcome to OpenBIOS v1.1 built on Jun 20 2014 22:58
0 > cd /cpus/PowerPC,750 ok
0 > .properties
name "PowerPC,750"
device_type "cpu"
cpu-version 80301
d-cache-size 8000
i-cache-size 8000
d-cache-sets 80
i-cache-sets 80
d-cache-block-size 20
i-cache-block-size 20
tlb-sets 40
tlb-size 80
timebase-frequency fd4bc0
clock-frequency fdad680
bus-frequency 3ef1480
state "running"
reg 00000000
available 00004000 07c54000
07e10000 f80f0000
00000000 ffffffff
translations 00001000 00003000 00001000 00000000
07c58000 001b8000 07c58000 00000000
fff00000 00100000 07f00000 00000000
ok
I'm not exactly sure how these frequencies affect the decrementer, but
it's enough to break OS X if these frequencies (particularly timebase
and clock) are not correct.
HTH,
Mark.
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