Module Name: src Committed By: riastradh Date: Tue Jul 16 22:45:10 UTC 2024
Modified Files: src/common/lib/libc/arch/x86_64/atomic: atomic.S Log Message: amd64: Fix performance regression in uniprocessor atomics/membars. Back in 2022, I eliminated the MFENCE hotpatch in membar_sync because it's essentially always more expensive than LOCK ADD with no benefit for CPU/CPU store-before-load ordering. (It is relevant only for non-temporal stores or write-combining memory.) https://mail-index.netbsd.org/source-changes/2022/07/30/msg140047.html But in that change, I made a mistake and _also_ eliminated the LOCK hotpatch on uniprocessor amd64. And our assembler gas helpfully interprets uppercase LOCK just like lowercase lock and assembles them the same way, so I didn't notice. This change restores the LOCK hotpatch, so that when booting on a uniprocessor system (or a uniprocessor guest on a multicore host), the LOCK prefix is replaced by NOP for a cheaper instruction. Found by puzzling over how my explanation for PR kern/57199 could possibly be correct when (on an amd64 guest) ddb x/i membar_sync kept showing the lock prefix even in uniprocessor boots. To generate a diff of this commit: cvs rdiff -u -r1.30 -r1.31 src/common/lib/libc/arch/x86_64/atomic/atomic.S Please note that diffs are not public domain; they are subject to the copyright notices on the relevant files.