Re: [PATCH v5 6/8] RISC-V: Use Zicboz in clear_page when available

Ben Dooks
2023-02-24T14:01:00Z

On 21/02/2023 19:09, Andrew Jones wrote:
> Using memset() to zero a 4K page takes 563 total instructions, where
> 20 are branches. clear_page(), with Zicboz and a 64 byte block size,
> takes 169 total instructions, where 4 are branches and 33 are nops.
> Even though the block size is a variable, thanks to alternatives, we
> can still implement a Duff device without having to do any preliminary
> calculations. This is achieved by using the alternatives' cpufeature
> value (the upper 16 bits of patch_id). The value used is the maximum
> zicboz block size order accepted at the patch site. This enables us
> to stop patching / unrolling when 4K bytes have been zeroed (we would
> loop and continue after 4K if the page size would be larger)
>
> For 4K pages, unrolling 16 times allows block sizes of 64 and 128 to
> only loop a few times and larger block sizes to not loop at all. Since
> cbo.zero doesn't take an offset, we also need an 'add' after each
> instruction, making the loop body 112 to 160 bytes. Hopefully this
> is small enough to not cause icache misses.
>
> Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
> Acked-by: Conor Dooley <conor.dooley@microchip.com>
> ---
> arch/riscv/Kconfig | 13 ++++++
> arch/riscv/include/asm/insn-def.h | 4 ++
> arch/riscv/include/asm/page.h | 6 ++-
> arch/riscv/kernel/cpufeature.c | 11 +++++
> arch/riscv/lib/Makefile | 1 +
> arch/riscv/lib/clear_page.S | 73 +++++++++++++++++++++++++++++++
> 6 files changed, 107 insertions(+), 1 deletion(-)
> create mode 100644 arch/riscv/lib/clear_page.S

[snip]

> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index 0594989ead63..4a496552b812 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -292,6 +292,17 @@ static bool riscv_cpufeature_patch_check(u16 id, u16 value)
> if (!value)
> return true;
>
> + switch (id) {
> + case RISCV_ISA_EXT_ZICBOZ:
> + /*
> + * Zicboz alternative applications provide the maximum
> + * supported block size order, or zero when it doesn't
> + * matter. If the current block size exceeds the maximum,
> + * then the alternative cannot be applied.
> + */
> + return riscv_cboz_block_size <= (1U << value);
> + }
> +
> return false;
> }
>
> diff --git a/arch/riscv/lib/Makefile b/arch/riscv/lib/Makefile
> index 6c74b0bedd60..26cb2502ecf8 100644
> --- a/arch/riscv/lib/Makefile
> +++ b/arch/riscv/lib/Makefile
> @@ -8,5 +8,6 @@ lib-y += strlen.o
> lib-y += strncmp.o
> lib-$(CONFIG_MMU) += uaccess.o
> lib-$(CONFIG_64BIT) += tishift.o
> +lib-$(CONFIG_RISCV_ISA_ZICBOZ) += clear_page.o
>
> obj-$(CONFIG_FUNCTION_ERROR_INJECTION) += error-inject.o
> diff --git a/arch/riscv/lib/clear_page.S b/arch/riscv/lib/clear_page.S
> new file mode 100644
> index 000000000000..7c7fa45b5ab5
> --- /dev/null
> +++ b/arch/riscv/lib/clear_page.S
> @@ -0,0 +1,73 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Copyright (c) 2023 Ventana Micro Systems Inc.
> + */
> +
> +#include <linux/linkage.h>
> +#include <asm/asm.h>
> +#include <asm/alternative-macros.h>
> +#include <asm/hwcap.h>
> +#include <asm/insn-def.h>
> +#include <asm/page.h>
> +
> +#define CBOZ_ALT(order, old, new) \
> + ALTERNATIVE(old, new, 0, \
> + ((order) << 16) | RISCV_ISA_EXT_ZICBOZ, \
> + CONFIG_RISCV_ISA_ZICBOZ)
> +
> +/* void clear_page(void *page) */
> +ENTRY(__clear_page)
> +WEAK(clear_page)

out of interest, why the __clear_page() entry and the
WEAK(clear_page)?

Just followed up with a patch to fix the modpost.

So far this seems to be working with qemu and a backport to 5.19.x

--
Ben Dooks http://www.codethink.co.uk/
Senior Engineer Codethink - Providing Genius