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> I think I just didn't leave an R-b last time because of the ongoing discussion & the functions naming. Given the conversation I attempted to summarise at [1], I'm satisfied that we're not just introducing something that'd conflict with trying to use this mechanism for non-extension capabilities: Acked-by: Conor Dooley <conor.dooley@xxxxxxxxxxxxx> 1 - https://lore.kernel.org/linux-riscv/Y8hqptFcUgjhns4F@spud/ > --- > arch/riscv/include/asm/hwcap.h | 37 ++++++++++++++++++++++++++++++++++ > 1 file changed, 37 insertions(+) > > diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h > index 09a7767723f6..1767a9ce1a04 100644 > --- a/arch/riscv/include/asm/hwcap.h > +++ b/arch/riscv/include/asm/hwcap.h > @@ -8,6 +8,7 @@ > #ifndef _ASM_RISCV_HWCAP_H > #define _ASM_RISCV_HWCAP_H > > +#include <asm/alternative-macros.h> > #include <asm/errno.h> > #include <linux/bits.h> > #include <uapi/asm/hwcap.h> > @@ -97,6 +98,42 @@ static __always_inline int riscv_isa_ext2key(int num) > } > } > > +static __always_inline bool > +riscv_has_extension_likely(const unsigned long ext) > +{ > + compiletime_assert(ext < RISCV_ISA_EXT_MAX, > + "ext must be < RISCV_ISA_EXT_MAX"); > + > + asm_volatile_goto( > + ALTERNATIVE("j %l[l_no]", "nop", 0, %[ext], 1) > + : > + : [ext] "i" (ext) > + : > + : l_no); > + > + return true; > +l_no: > + return false; > +} > + > +static __always_inline bool > +riscv_has_extension_unlikely(const unsigned long ext) > +{ > + compiletime_assert(ext < RISCV_ISA_EXT_MAX, > + "ext must be < RISCV_ISA_EXT_MAX"); > + > + asm_volatile_goto( > + ALTERNATIVE("nop", "j %l[l_yes]", 0, %[ext], 1) > + : > + : [ext] "i" (ext) > + : > + : l_yes); > + > + return false; > +l_yes: > + return true; > +} > + > unsigned long riscv_isa_extension_base(const unsigned long *isa_bitmap); > > #define riscv_isa_extension_mask(ext) BIT_MASK(RISCV_ISA_EXT_##ext) > -- > 2.38.1 > > > _______________________________________________ > linux-riscv mailing list > linux-riscv@xxxxxxxxxxxxxxxxxxx > http://lists.infradead.org/mailman/listinfo/linux-riscv
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