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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH 00/16] make system memory API available for common code
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. 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. 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. 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. In all cases, transforming code relying on compile time optimization/dead code elimination through defines to runtime checks will *always* have an impact, even though it should be minimal in most of cases. 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.Regards, BALATON ZoltanRegards, 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 (roughly in this order) on: 1. quality of code generated by TCG 2. helper code to implement instructions 3. mmu emulationOther 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. There are probably some other low hanging fruits for other target architectures. [1] https://www.linaro.org/blog/qemu-a-tale-of-performance-analysis/
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