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 hCjxh6PI 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000009, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Mateusz Guzik writes: > On Mon, Apr 14, 2025 at 01:02:59PM +0200, Peter Zijlstra wrote: >> This symbol is written as a C function with C calling convention, even >> though it is only meant to be called from that clear_page() alternative. >> >> If we want to go change all this, then we should go do the same we do >> for __clear_user() and write it thusly: >> >> asm volatile(ALTERNATIVE("rep stosb", >> "call rep_stos_alternative", ALT_NOT(X86_FEATURE_FSRS) >> : "+c" (size), "+D" (addr), ASM_CALL_CONSTRAINT >> : "a" (0)) >> >> And forget about all those clear_page_*() thingies. >> > > I have to disagree. > > Next to nobody has FSRS, so for now one would have to expect everyone > would be punting to the routine. Did you mean ERMS as sizes are in fact > not short? > > rep_stos_alternative() as implemented right now sucks in its own right > ("small" areas sorted out with an 8 byte and 1 byte loops, bigger ones > unrolled 64 byte loop at a time, no rep stos{b,q} in sight). Someone(tm) > should fix it and for the sake of argument suppose it happened. That's > still some code executed to figure out how to zero and to align the buf. > > Instead, I think one can start with just retiring clear_page_orig(). > > With that sucker out of the way, an optional quest is to figure out if > rep stosq vs rep stosb makes any difference for pages -- for all I know > rep stosq is the way. This would require testing on quite a few uarchs > and I'm not going to blame anyone for not being interested. IIRC some recent AMD models (Rome?) did expose REP_GOOD but not ERMS. > Let's say nobody bothered OR rep stosb provides a win. In that case this > can trivially ALTERNATIVE between rep stosb and rep stosq based on ERMS, > no func calls necessary. We shouldn't need any function calls for ERMS and REP_GOOD. I think something like this untested code should work: asm volatile( ALTERNATIVE_2("call clear_pages_orig", "rep stosb", X86_FEATURE_REP_GOOD, "shrl $3,%ecx; rep stosq", X86_FEATURE_ERMS, : "+c" (size), "+D" (addr), ASM_CALL_CONSTRAINT : "a" (0))) -- ankur