On Mon, 10 Mar 2025, Pierrick Bouvier wrote:
On 3/10/25 09:28, Pierrick Bouvier wrote:
Hi Zoltan,
On 3/10/25 06:23, BALATON Zoltan wrote:
On Sun, 9 Mar 2025, Pierrick Bouvier wrote:
The main goal of this series is to be able to call any memory ld/st
function
from code that is *not* target dependent.
Why is that needed?
this series belongs to the "single binary" topic, where we are trying to
build a single QEMU binary with all architectures embedded.
Yes I get it now, I just forgot as this wasn't mentioned so the goal
wasn't obvious.
To achieve that, we need to have every single compilation unit compiled
only once, to be able to link a binary without any symbol conflict.
A consequence of that is target specific code (in terms of code relying
of target specific macros) needs to be converted to common code,
checking at runtime properties of the target we run. We are tackling
various places in QEMU codebase at the same time, which can be confusing
for the community members.
Mentioning this single binary in related series may help reminding readers
about the context.
This series take care of system memory related functions and associated
compilation units in system/.
As a positive side effect, we can
turn related system compilation units into common code.
Are there any negative side effects? In particular have you done any
performance benchmarking to see if this causes a measurable slow down?
Such as with the STREAM benchmark:
https://stackoverflow.com/questions/56086993/what-does-stream-memory-bandwidth-benchmark-really-measure
Maybe it would be good to have some performance tests similiar to
functional tests that could be run like the CI tests to detect such
performance changes. People report that QEMU is getting slower and slower
with each release. Maybe it could be a GSoC project to make such tests but
maybe we're too late for that.
I agree with you, and it's something we have mentioned during our
"internal" conversations. Testing performance with existing functional
tests would already be a first good step. However, given the poor
reliability we have on our CI runners, I think it's a bit doomed.
Ideally, every QEMU release cycle should have a performance measurement
window to detect potential sources of regressions.
Maybe instead of aiming for full CI like performance testing something
simpler like a few tests that excercise some apects each like STREAM that
tests memory access, copying a file from network and/or disk that tests
I/O and mp3 encode with lame for example that's supposed to test floating
point and SIMD might be simpler to do. It could be made a bootable image
that just runs the test and reports a number (I did that before for
qemu-system-ppc when we wanted to test an issue that on some hosts it ran
slower). Such test could be run by somebody making changes so they could
call these before and after their patch to quickly check if there's
anything to improve. This may be less through then full performance
testing but still give some insight and better than not testing anything
for performance.
I'm bringig this topic up to try to keep awareness on this so QEMU can
remain true to its name. (Although I'm not sure if originally the Q in the
name stood for the time it took to write or its performance but it's
hopefully still a goal to keep it fast.)
To answer to your specific question, I am trying first to get a review
on the approach taken. We can always optimize in next series version, in
case we identify it's a big deal to introduce a branch for every memory
related function call.
I'm not sure we can always optimise after the fact so sometimes it can be
necessary to take performance in consideration while designing changes.
In all cases, transforming code relying on compile time
optimization/dead code elimination through defines to runtime checks
will *always* have an impact,
Yes, that's why it would be good to know how much impact is that.
even though it should be minimal in most of cases.
Hopefully but how do we know if we don't even test for it?
But the maintenance and compilation time benefits, as well as
the perspectives it opens (single binary, heterogeneous emulation, use
QEMU as a library) are worth it IMHO.
I'm not so sure about that. Heterogeneous emulation sounds interesting but
is it needed most of the time? Using QEMU as a library also may not be
common and limited by licencing. The single binary would simplify packages
but then this binary may get huge so it's slower to load, may take more
resources to run and more time to compile and if somebody only needs one
architecture why do I want to include all of the others and wait for it to
compile using up a lot of space on my disk? So in other words, while these
are interesting and good goals could it be achieved with keeping the
current way of building single ARCH binary as opposed to single binary
with multiple archs and not throwing out the optimisations a single arch
binary can use? Which one is better may depend on the use case so if
possible it would be better to allow both keeping what we have and adding
multi arch binary on top not replacing the current way completely.
Regards,
BALATON Zoltan
Regards,
Pierrick
As a side note, we recently did some work around performance analysis (for
aarch64), as you can see here [1]. In the end, QEMU performance depends
Thank you, very interesting read.
(roughly in this order) on:
1. quality of code generated by TCG
2. helper code to implement instructions
3. mmu emulation
Other state of the art translators that exist are faster (fex, box64) mainly
by enhancing 1, and relying on various tricks to avoid translating some
libraries calls. But those translators are host/target specific, and the
ratio of instructions generated (vs target ones read) is much lower than
QEMU. In the experimentation listed in the blog, I observed that for
qemu-system-aarch64, we have an average expansion factor of around 18 (1
guest insn translates to 18 host ones).
For users seeing performance decreases, beyond the QEMU code changes, adding
new target instructions may add new helpers, which may be called by the stack
people use, and they can sometimes observe a slower behaviour.
I'm mostly interested in emulating PPC for older and obscure OSes running
on older hardware so there new instructions isn't a problem. Most of the
time MMU emulation, helpers and TCG code generation is mostly dominating
there and on PPC particularly the lack of hard float usage. Apart from
that maybe some device emulations but that's a different topic. This is
already slow so any overhead introduced at lowest levels just adds to
that and target specific optimisation may only get back what's lost
elsewhere.
Regards,
BALATON Zoltan
There are probably some other low hanging fruits for other target
architectures.
[1] https://www.linaro.org/blog/qemu-a-tale-of-performance-analysis/
