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9:24=E2=80=AFPM Ankur Arora wrote: >> + * Switch between three implementations of page clearing based on CPU >> + * capabilities: >> + * >> + * - __clear_pages_unrolled(): the oldest, slowest and universally >> + * supported method. Zeroes via 8-byte MOV instructions unrolled 8x >> + * to write a 64-byte cacheline in each loop iteration. >> + * >> + * - "REP; STOSQ": really old CPUs had crummy REP implementations. >> + * Vendor CPU setup code sets 'REP_GOOD' on CPUs where REP can be >> + * trusted. The instruction writes 8-byte per REP iteration but >> + * CPUs can internally batch these together and do larger writes. >> + * >> + * - "REP; STOSB": CPUs that enumerate 'ERMS' have an improved STOS >> + * implementation that is less picky about alignment and where >> + * STOSB (1-byte at a time) is actually faster than STOSQ (8-bytes >> + * at a time.) >> + * > > I think this is somewhat odd commentary in this context. > > Note about "crummy REP implementations" should be in description of > __clear_pages_unrolled as it justifies its existence (I think the > routine would be best whacked btw, but I'm not going to argue about it > in this thread). > Description of STOSQ notes the CPU can do more than 8 bytes at a time, > while description of STOSB claim does not make such a clarification. > At the same time the note about less picky about alignment makes no > significance in the context of page clearing as they are, well, page > aligned. Good point. I'll rework the comment a little bit to align things better (maybe reusing some of what you suggest below). > There is a fucky real-world problem with ERMS worth noting: there are > hypervisor setups out there which *hide* the bit by default (no > really, see Proxmox for example -- you get a bare bones pre-ERMS > cpuid) > > With all this in mind, modulo poor grammar on my end, I would suggest > something like this: > > > There are 3 variants implemented: > - REP; STOSB: used if the CPU supports "Enhanced REP MOVSB/STOSB" (aka > ERMS), which is true for majority of microarchitectures today > - REP; STOSQ: fallback if the ERMS bit is not present > - __clear_pages_unrolled: code for CPUs which are determined to have > poor REP support, only concerns long obsolete uarchs. > > Warnings: some hypervisors are configured to expose a very limited set > of capabilites in the guest, fitering out ERMS even if present. As > such the STOSQ variant is still in active use on some setups even when > hardware does not need it. > The last bit is useful context though maybe some of it fits better in the commit message. Thanks ankur