Messages in this thread | | | From | "Luck, Tony" <> | Subject | RE: [PATCH] x86: also disable FSRM if ERMS is disabled | Date | Tue, 11 Oct 2022 19:08:51 +0000 |
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> If something clears MSR_IA32_MISC_ENABLE_FAST_STRING_BIT and we go and > clear our feature flags and luserspace still queries CPUID then oh well, > it'll be fun. It all depends on why something has cleared them tho. It > could be some performance thing or something a lot more funky. I guess > if stuff starts exploding left and right, there will soon be a microcode > patch after that. :-)
Yes. ERMS CPUID bit was the early indicator to s/w that REP MOVS would do some fancy speedups if certain conditions about source, destination and count are all met. A hint to s/w to decide which memcpy() routine to use.
FSRM is a hint that the byte count restriction had pretty much been removed so s/w can use REP MOVS in even more places.
I don't think Intel will deliberately release a CPU that has FSRM=1, ERMS=0.
-Tony
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