| Message ID | 20250310045842.2650784-1-pierrick.bouvier@linaro.org (mailing list archive) |
|---|---|
| Headers | show |
| Series | make system memory API available for common code | expand |
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? > 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. Regards, BALATON Zoltan
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 Zoltan Regards, Pierrick
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 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 (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. There are probably some other low hanging fruits for other target architectures. [1] https://www.linaro.org/blog/qemu-a-tale-of-performance-analysis/
The main goal of this series is to be able to call any memory ld/st function from code that is *not* target dependent. As a positive side effect, we can turn related system compilation units into common code. The first 6 patches remove dependency of memory API to cpu headers and remove dependency to target specific code. This could be a series on its own, but it's great to be able to turn system memory compilation units into common code to make sure it can't regress, and prove it achieves the desired result. The next patches remove more dependencies on cpu headers (exec-all, memory-internal, ram_addr). Then, we add access to a needed function from kvm, some xen stubs, and we finally can turn our compilation units into common code. Every commit was tested to build correctly for all targets (on windows, linux, macos), and the series was fully tested by running all tests we have (linux, x86_64 host). Pierrick Bouvier (16): exec/memory_ldst: extract memory_ldst declarations from cpu-all.h exec/memory_ldst_phys: extract memory_ldst_phys declarations from cpu-all.h include: move target_words_bigendian() from tswap to bswap exec/memory.h: make devend_memop target agnostic qemu/bswap: implement {ld,st}.*_p as functions exec/cpu-all.h: we can now remove ld/st macros codebase: prepare to remove cpu.h from exec/exec-all.h exec/exec-all: remove dependency on cpu.h exec/memory-internal: remove dependency on cpu.h exec/ram_addr: remove dependency on cpu.h system/kvm: make kvm_flush_coalesced_mmio_buffer() accessible for common code exec/ram_addr: call xen_hvm_modified_memory only if xen is enabled hw/xen: add stubs for various functions system/physmem: compilation unit is now common to all targets system/memory: make compilation unit common system/ioport: make compilation unit common include/exec/cpu-all.h | 52 ------------------ include/exec/exec-all.h | 1 - include/exec/memory-internal.h | 2 - include/exec/memory.h | 48 ++++++++++++++--- include/exec/ram_addr.h | 11 ++-- include/exec/tswap.h | 11 ---- include/qemu/bswap.h | 82 +++++++++++++++++++++++++++++ include/system/kvm.h | 6 +-- include/tcg/tcg-op.h | 1 + target/ppc/helper_regs.h | 2 + include/exec/memory_ldst.h.inc | 13 ++--- include/exec/memory_ldst_phys.h.inc | 5 +- hw/ppc/spapr_nested.c | 1 + hw/sh4/sh7750.c | 1 + hw/xen/xen_stubs.c | 56 ++++++++++++++++++++ page-vary-target.c | 3 +- system/ioport.c | 1 - system/memory.c | 22 +++++--- target/riscv/bitmanip_helper.c | 1 + hw/xen/meson.build | 3 ++ system/meson.build | 6 +-- 21 files changed, 225 insertions(+), 103 deletions(-) create mode 100644 hw/xen/xen_stubs.c