Re: [PATCH] RISC-V: Optimize bitops with Zbb extension

From: Conor Dooley
Date: Wed Aug 30 2023 - 15:03:33 EST

On Wed, Aug 30, 2023 at 06:14:12AM +0000, Wang, Xiao W wrote:
> Hi,
>
> > -----Original Message-----
> > From: Anup Patel <anup@xxxxxxxxxxxxxx>
> > Sent: Tuesday, August 29, 2023 7:08 PM
> > To: Wang, Xiao W <xiao.w.wang@xxxxxxxxx>
> > Cc: paul.walmsley@xxxxxxxxxx; palmer@xxxxxxxxxxx;
> > aou@xxxxxxxxxxxxxxxxx; ardb@xxxxxxxxxx; Li, Haicheng
> > <haicheng.li@xxxxxxxxx>; linux-riscv@xxxxxxxxxxxxxxxxxxx; linux-
> > efi@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> > Subject: Re: [PATCH] RISC-V: Optimize bitops with Zbb extension
> >
> > On Sun, Aug 6, 2023 at 8:09 AM Xiao Wang <xiao.w.wang@xxxxxxxxx> wrote:
> > >
> > > This patch leverages the alternative mechanism to dynamically optimize
> > > bitops (including __ffs, __fls, ffs, fls) with Zbb instructions. When
> > > Zbb ext is not supported by the runtime CPU, legacy implementation is
> > > used. If Zbb is supported, then the optimized variants will be selected
> > > via alternative patching.
> > >
> > > The legacy bitops support is taken from the generic C implementation as
> > > fallback.
> > >
> > > If the parameter is a build-time constant, we leverage compiler builtin to
> > > calculate the result directly, this approach is inspired by x86 bitops
> > > implementation.
> > >
> > > EFI stub runs before the kernel, so alternative mechanism should not be
> > > used there, this patch introduces a macro EFI_NO_ALTERNATIVE for this
> > > purpose.
> >
> > I am getting the following compile error with this patch:
> >
> > GEN Makefile
> > UPD include/config/kernel.release
> > UPD include/generated/utsrelease.h
> > CC kernel/bounds.s
> > In file included from /home/anup/Work/riscv-
> > test/linux/include/linux/bitmap.h:9,
> > from
> > /home/anup/Work/riscv-test/linux/arch/riscv/include/asm/cpufeature.h:9,
> > from
> > /home/anup/Work/riscv-test/linux/arch/riscv/include/asm/hwcap.h:90,
>
>
> It looks there's a cyclic header including, which leads to this build error.
> I checked https://github.com/kvm-riscv/linux/tree/master and
> https://github.com/torvalds/linux/tree/master, but I don't see
> "asm/cpufeature.h" is included in asm/hwcap.h:90, maybe I miss something,
> could you help point me to the repo/branch I should work on?

From MAINTAINERS:
RISC-V ARCHITECTURE
...
T: git git://git.kernel.org/pub/scm/linux/kernel/git/riscv/linux.git

The for-next branch there is what you should be basing work on top of.
AFAICT, you've made bitops.h include hwcap.h while cpufeature.h includes
both bitops.h (indirectly) and hwcap.h.

Hope that helps,
Conor.

> > from
> > /home/anup/Work/riscv-test/linux/arch/riscv/include/asm/bitops.h:26,
> > from
> > /home/anup/Work/riscv-test/linux/include/linux/bitops.h:68,
> > from /home/anup/Work/riscv-test/linux/include/linux/log2.h:12,
> > from /home/anup/Work/riscv-test/linux/kernel/bounds.c:13:
> > /home/anup/Work/riscv-test/linux/include/linux/find.h: In function
> > 'find_next_bit':
> > /home/anup/Work/riscv-test/linux/include/linux/find.h:64:30: error:
> > implicit declaration of function '__ffs'
> > [-Werror=implicit-function-declaration]
> > 64 | return val ? __ffs(val) : size;
> >
> > Regards,
> > Anup
> >
> >
> > >
> > > Signed-off-by: Xiao Wang <xiao.w.wang@xxxxxxxxx>
> > > ---
> > > arch/riscv/include/asm/bitops.h | 266 +++++++++++++++++++++++++-
> > > drivers/firmware/efi/libstub/Makefile | 2 +-
> > > 2 files changed, 264 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/arch/riscv/include/asm/bitops.h
> > b/arch/riscv/include/asm/bitops.h
> > > index 3540b690944b..f727f6489cd5 100644
> > > --- a/arch/riscv/include/asm/bitops.h
> > > +++ b/arch/riscv/include/asm/bitops.h
> > > @@ -15,13 +15,273 @@
> > > #include <asm/barrier.h>
> > > #include <asm/bitsperlong.h>
> > >
> > > +#if !defined(CONFIG_RISCV_ISA_ZBB) || defined(EFI_NO_ALTERNATIVE)
> > > #include <asm-generic/bitops/__ffs.h>
> > > -#include <asm-generic/bitops/ffz.h>
> > > -#include <asm-generic/bitops/fls.h>
> > > #include <asm-generic/bitops/__fls.h>
> > > +#include <asm-generic/bitops/ffs.h>
> > > +#include <asm-generic/bitops/fls.h>
> > > +
> > > +#else
> > > +#include <asm/alternative-macros.h>
> > > +#include <asm/hwcap.h>
> > > +
> > > +#if (BITS_PER_LONG == 64)
> > > +#define CTZW "ctzw "
> > > +#define CLZW "clzw "
> > > +#elif (BITS_PER_LONG == 32)
> > > +#define CTZW "ctz "
> > > +#define CLZW "clz "
> > > +#else
> > > +#error "Unexpected BITS_PER_LONG"
> > > +#endif
> > > +
> > > +static __always_inline unsigned long variable__ffs(unsigned long word)
> > > +{
> > > + int num;
> > > +
> > > + asm_volatile_goto(
> > > + ALTERNATIVE("j %l[legacy]", "nop", 0, RISCV_ISA_EXT_ZBB, 1)
> > > + : : : : legacy);
> > > +
> > > + asm volatile (
> > > + ".option push\n"
> > > + ".option arch,+zbb\n"
> > > + "ctz %0, %1\n"
> > > + ".option pop\n"
> > > + : "=r" (word) : "r" (word) :);
> > > +
> > > + return word;
> > > +
> > > +legacy:
> > > + num = 0;
> > > +#if BITS_PER_LONG == 64
> > > + if ((word & 0xffffffff) == 0) {
> > > + num += 32;
> > > + word >>= 32;
> > > + }
> > > +#endif
> > > + if ((word & 0xffff) == 0) {
> > > + num += 16;
> > > + word >>= 16;
> > > + }
> > > + if ((word & 0xff) == 0) {
> > > + num += 8;
> > > + word >>= 8;
> > > + }
> > > + if ((word & 0xf) == 0) {
> > > + num += 4;
> > > + word >>= 4;
> > > + }
> > > + if ((word & 0x3) == 0) {
> > > + num += 2;
> > > + word >>= 2;
> > > + }
> > > + if ((word & 0x1) == 0)
> > > + num += 1;
> > > + return num;
> > > +}
> > > +
> > > +/**
> > > + * __ffs - find first set bit in a long word
> > > + * @word: The word to search
> > > + *
> > > + * Undefined if no set bit exists, so code should check against 0 first.
> > > + */
> > > +#define __ffs(word) \
> > > + (__builtin_constant_p(word) ? \
> > > + (unsigned long)__builtin_ctzl(word) : \
> > > + variable__ffs(word))
> > > +
> [...]

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