Re: [PATCH v5 5/8] riscv/runtime-const: Introduce runtime_const_mask_32()

[Charlie Jenkins]

On Fri, Jul 10, 2026 at 01:47:10PM +0530, K Prateek Nayak wrote:
> Hello Charlie,
> 
> On 7/10/2026 1:22 PM, Charlie Jenkins wrote:
> > [You don't often get email from thecharlesjenkins@xxxxxxxxx. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ]
> > 
> > On Tue, 30 Jun 2026 04:55:28 +0000, K Prateek Nayak <kprateek.nayak@xxxxxxx> wrote:
> >> Futex hash computation requires a mask operation with read-only after
> >> init data that will be converted to a runtime constant in the subsequent
> >> commit.
> >>
> >> Introduce runtime_const_mask_32 to further optimize the mask operation
> >> in the futex hash computation hot path. Since all the current use-cases
> >> are of the form GENMASK(n, 0), with n > 0, following sequence:
> > 
> > I really appreciate you spending the time to do this, thank you!
> 
> My pleasure! And I really appreciate you taking time to review and test
> this series. Thanks a ton for that!
> 
> > 
> >>
> >>
> >> diff --git a/arch/riscv/include/asm/runtime-const.h b/arch/riscv/include/asm/runtime-const.h
> >> index 1ce02605d2e4..dbf96c937dbb 100644
> >> --- a/arch/riscv/include/asm/runtime-const.h
> >> +++ b/arch/riscv/include/asm/runtime-const.h
> >> @@ -262,6 +279,33 @@ static inline void __runtime_fixup_shift(void *where, unsigned long val)
> >> [ ... skip 24 lines ... ]
> >> +     BUG_ON(!val || width > 31 || (GENMASK(width - 1, 0) != val));
> >> +
> >> +     __runtime_fixup_shift(where, 32 - width);
> >> +     __runtime_fixup_shift(where + 4, 32 - width);
> >> +}
> >> +
> > 
> > It would be "optimal" to use an andi when the mask is <=11 bits since
> > andi can fit an 11 bit mask. What you have is good enough but I'll leave
> > my stab at doing the andi patching here in case you want to apply it.
> > 
> > From 9e5527aaddd464783af795aacdb6d094e11cc31e Mon Sep 17 00:00:00 2001
> > From: Charlie Jenkins <thecharlesjenkins@xxxxxxxxx>
> > Date: Thu, 9 Jul 2026 23:18:09 -0700
> > Subject: [PATCH] riscv: Optimize __runtime_fixup_mask for masks with <= 11
> >  bits
> 
> Peter seems to have merged the v5 series in his tree but If you could give
> your S-o-b, I can throw in a commit log, some testing along with a few
> cosmetic modifications, and send it for official review on top of
> queue:locking/core ;-)

I'm glad it got merged, I missed that! Here's my tag that I forgot to
add...

Signed-off-by: Charlie Jenkins <thecharlesjenkins@xxxxxxxxx>

> 
> > 
> > ---
> >  arch/riscv/include/asm/insn.h          |  2 ++
> >  arch/riscv/include/asm/runtime-const.h | 29 ++++++++++++++++++++++++--
> >  2 files changed, 29 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/riscv/include/asm/insn.h b/arch/riscv/include/asm/insn.h
> > index c3005573e8c9..0a34cd7305d0 100644
> > --- a/arch/riscv/include/asm/insn.h
> > +++ b/arch/riscv/include/asm/insn.h
> > @@ -141,6 +141,7 @@
> >  #define RVG_OPCODE_JALR                0x67
> >  #define RVG_OPCODE_JAL         0x6f
> >  #define RVG_OPCODE_SYSTEM      0x73
> > +#define RVG_OPCODE_ANDI                0x13
> >  #define RVG_SYSTEM_CSR_OFF     20
> >  #define RVG_SYSTEM_CSR_MASK    GENMASK(12, 0)
> > 
> > @@ -175,6 +176,7 @@
> >  #define RVG_FUNCT3_BGE         0x5
> >  #define RVG_FUNCT3_BLTU                0x6
> >  #define RVG_FUNCT3_BGEU                0x7
> > +#define RVG_FUNCT3_ANDI                0x7
> > 
> >  /* parts of funct3 code for C extension*/
> >  #define RVC_FUNCT3_C_BEQZ      0x6
> > diff --git a/arch/riscv/include/asm/runtime-const.h b/arch/riscv/include/asm/runtime-const.h
> > index dbf96c937dbb..24a9b13081f7 100644
> > --- a/arch/riscv/include/asm/runtime-const.h
> > +++ b/arch/riscv/include/asm/runtime-const.h
> > @@ -9,6 +9,7 @@
> >  #include <asm/asm.h>
> >  #include <asm/alternative.h>
> >  #include <asm/cacheflush.h>
> > +#include <asm/insn.h>
> >  #include <asm/insn-def.h>
> >  #include <linux/memory.h>
> >  #include <asm/text-patching.h>
> > @@ -302,8 +303,32 @@ static inline void __runtime_fixup_mask(void *where, unsigned long val)
> >          */
> >         BUG_ON(!val || width > 31 || (GENMASK(width - 1, 0) != val));
> > 
> > -       __runtime_fixup_shift(where, 32 - width);
> > -       __runtime_fixup_shift(where + 4, 32 - width);
> > +       /*
> > +        * A riscv 'andi' instruction can fit an 11 bit immediate, so the mask
> > +        * can be directly applied. Otherwise fall back to SRLI + SLLI.
> > +        */
> > +       if (width < 11) {
> > +               __le16 *parcel = where;
> > +               u32 insn;
> > +               __le32 res, nop;
> > +
> > +               insn = (u32)le16_to_cpu(parcel[0]) | (u32)le16_to_cpu(parcel[1]) << 16;
> > +
> > +               /* Replace the slli/slliw with an andi */
> > +               insn &= 0x000fcf80;
> > +               insn |= val << 20 | RV_ENCODE_FUNCT3(ANDI) | RVG_OPCODE_ANDI;
> > +
> > +               res = cpu_to_le32(insn);
> > +               /* Replace the srli/srliw with a nop */
> > +               nop = cpu_to_le32(RISCV_INSN_NOP4);
> > +               mutex_lock(&text_mutex);
> > +               patch_text_nosync(where, &res, sizeof(insn));
> > +               patch_text_nosync(where + 4, &nop, sizeof(insn));
> > +               mutex_unlock(&text_mutex);
> > +       } else {
> > +               __runtime_fixup_shift(where, 32 - width);
> > +               __runtime_fixup_shift(where + 4, 32 - width);
> > +       }
> >  }
> > 
> >  static inline void runtime_const_fixup(void (*fn)(void *, unsigned long),
> > --
> > 2.54.0
> > 
> > 
> > I would prefer including this, but I am happy to approve this
> > regardless.
> 
> Ack! I'll keep it as an optimization on top to retain your attribution.
> 
> > 
> > Reviewed-by: Charlie Jenkins <thecharlesjenkins@xxxxxxxxx>
> > Tested-by: Charlie Jenkins <thecharlesjenkins@xxxxxxxxx>
> 
> Thank you again!
> 
> -- 
> Thanks and Regards,
> Prateek
>