Generally, riscv ISA extensions are fixed for any specific hardware
platform, that's to say, the hart features won't change any more
after booting, this chacteristic make it straightforward to use
static branch to check one 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.
[1]https://lore.kernel.org/linux-riscv/[email protected]/
[2]https://lore.kernel.org/linux-arm-kernel/[email protected]/
Signed-off-by: Jisheng Zhang <[email protected]>
Reviewed-by: Andrew Jones <[email protected]>
---
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 996884986fea..e2d3f6df7701 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>
@@ -96,6 +97,42 @@ 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)
--
2.37.2
Hey Jisheng,
Just a couple really minor bits here...
On Mon, Dec 05, 2022 at 01:46:25AM +0800, Jisheng Zhang wrote:
> Generally, riscv ISA extensions are fixed for any specific hardware
> platform, that's to say, the hart features won't change any more
s/that's to say, the hart/so a hart's/
s/any more//
> after booting, this chacteristic make it straightforward to use
"booting. This characteristic makes it"
> static branch to check one specific ISA extension is supported or not
"a static branch to check if a"
> 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.
>
> [1]https://lore.kernel.org/linux-riscv/[email protected]/
> [2]https://lore.kernel.org/linux-arm-kernel/[email protected]/
Can you make these into Link: tags please (and drop the line between the
and the SoB)? So:
Link: https://lore.kernel.org/linux-riscv/[email protected]/ [1]
Link: https://lore.kernel.org/linux-arm-kernel/[email protected]/ [2]
> Signed-off-by: Jisheng Zhang <[email protected]>
> Reviewed-by: Andrew Jones <[email protected]>
Changes themselves look grand, no comments there :)
Thanks!
Conor.
> ---
> 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 996884986fea..e2d3f6df7701 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>
> @@ -96,6 +97,42 @@ 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)
> --
> 2.37.2
>