On 3/15/2023 2:15 AM, Richard Henderson wrote: > On 3/14/23 06:47, Wu, Fei wrote: >> On 3/13/2023 11:00 PM, Richard Henderson wrote: >>> On 3/13/23 07:13, Wu, Fei2 wrote: >>>> Hi Richard, >>>> >>>> Sorry for disturbing you. I'm doing some perf profiling on >>>> qemu-riscv64, >>>> I see 10%+ faster to build stress-ng without the following patch. I >>>> know >>>> it's incorrect to just skip this patch, I'm wondering if we can do >>>> something on intercepting mmap/mprotect (very rare), e.g. even >>>> invalidating all the TBs, but keep the cross-page block chaining. >>> >>> It also affects breakpoints. >>> >>> I have no good ideas for how to keep cross-page block chaining without >>> breaking either of these use cases. If you come up with a good idea, >>> please post on qemu-devel for discussion. >>> >> Thank you for reply. I am new to qemu/tcg, lots of details and >> backgrounds need to catch up. >> >> If we only want to address user-mode qemu, and assume this cross-page >> chain, first page -> second page: >> >> * breakpoints. If a new bp is added to second page, the chain is hard to >> maintain, but it looks acceptable to flush all TBs and fall back to >> current non-cross-page implementation during debugging? I think It's >> different from the full system situation here: >> https://gitlab.com/qemu-project/qemu/-/issues/404 >> >> * mprotect. If the 2nd page remains 'X' permission after mprotect, the >> chain is still valid, if it's changed to non-X, then the syscall >> interceptor will change the permission of corresponding host page to >> non-X, it will be segfault as expected? >> >> * mmap. I cannot figure out the situation. Is there any unit test for >> this, or could you please shed some light? > Also munmap, but handled via the same path through page_set_flags, see > > if (inval_tb) { > tb_invalidate_phys_range(start, end); > } > > There is no unit test for mmap over an existing code page. > I believe we do have one for mprotect. > > You could plausibly add a global variable choosing between > link-all-pages and link-one-page modes; it would be protected by > mmap_lock. For link-all-pages mode, the above tb_invalidate_phys_range > becomes tb_flush. We probably want to start in link-one-page mode if > gdbstub is active, which is the only way to set breakpoints in user-only > mode. > > I expect mprotect/mmap over existing executable pages to be extremely > rare. I expect munmap of existing executable pages to be rare-ish, with > dlclose() being the most common case. You might wish to change from > link-all-pages mode to link-one-page mode after one or more instances. > > And as I said, this discussion should happen on qemu-devel. > My fault. I didn't notice the cc list, and initialized another thread: https://www.mail-archive.com/qemu-devel@nongnu.org/msg949625.html
Would you prefer commenting there, or I move the content here? Thanks, Fei. > > r~
