Hey Jisheng, On Sun, Jan 15, 2023 at 11:49:46PM +0800, Jisheng Zhang wrote: > Generally, riscv ISA extensions are fixed for any specific hardware > platform, so a hart's features won't change after booting. This > chacteristic makes it straightforward to use a static branch to check > if a specific ISA extension is supported or not to optimize > performance. > > However, some ISA extensions such as SVPBMT and ZICBOM are handled > via. the alternative sequences. > > Basically, for ease of maintenance, we prefer to use static branches > in C code, but recently, Samuel found that the static branch usage in > cpu_relax() breaks building with CONFIG_CC_OPTIMIZE_FOR_SIZE[1]. As > Samuel pointed out, "Having a static branch in cpu_relax() is > problematic because that function is widely inlined, including in some > quite complex functions like in the VDSO. A quick measurement shows > this static branch is responsible by itself for around 40% of the jump > table." > > Samuel's findings pointed out one of a few downsides of static branches > usage in C code to handle ISA extensions detected at boot time: > static branch's metadata in the __jump_table section, which is not > discarded after ISA extensions are finalized, wastes some space. > > I want to try to solve the issue for all possible dynamic handling of > ISA extensions at boot time. Inspired by Mark[2], this patch introduces > riscv_has_extension_*() helpers, which work like static branches but > are patched using alternatives, thus the metadata can be freed after > patching. > > Link: https://lore.kernel.org/linux-riscv/20220922060958.44203-1-samuel@xxxxxxxxxxxx/ [1] > Link: https://lore.kernel.org/linux-arm-kernel/20220912162210.3626215-8-mark.rutland@xxxxxxx/ [2] > Signed-off-by: Jisheng Zhang <jszhang@xxxxxxxxxx> > Reviewed-by: Andrew Jones <ajones@xxxxxxxxxxxxxxxx> It'd be great, if, in the future, you would hold off on sending new versions of patchsets where the previous version is still being discussed [3]. ~3 days between versions is not very much, especially when that includes a weekend! I know you replied there earlier today with your opinion, but please give people a chance to read and respond, before resubmitting, so as not to split discussion between several threads. 3 - https://lore.kernel.org/linux-riscv/2398293.3Lj2Plt8kZ@diego/ Thanks, Conor.
Attachment:
signature.asc
Description: PGP signature
