On 11/02/2025 8:53 pm, Dave Hansen wrote: > On 2/11/25 11:43, Sohil Mehta wrote: >> + /* >> + * Modern CPUs are generally expected to have a sane fast string >> + * implementation. However, the BIOS may disable it on certain CPUs >> + * via the architectural FAST_STRING bit. >> + */ >> + if (IS_ENABLED(CONFIG_X86_64) && (c->x86 == 6 || c->x86 > 15)) >> + set_cpu_cap(c, X86_FEATURE_REP_GOOD); > I'm not sure the BIOS comment is helpful here. > > Also, at this point, let's just make the check >=6 (or the >=PPRO > equivalent). > > It will only matter if *all* of these are true: > 1. Someone has a 64-bit capable P4 that powers on > 2. They're running a 64-bit mainline kernel > 3. String copy is *actually* slower than the alternative > 4. They are performance sensitive enough to notice > > We don't even know the answer to #3 for sure. Let's just say what we're > doing in a comment: > > /* Assume that any 64-bit CPU has a good implementation */ If you're going to override the BIOS setting, then you need to explicitly set MSR_MISC_ENABLE.FAST_STRINGS. Otherwise you're claiming to Linux that REP is good even when hardware is prohibited from using optimisations. ~Andrew
