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

Andrew Jones
2023-02-24T14:25:50Z

On Fri, Feb 24, 2023 at 02:00:44PM +0000, Ben Dooks wrote:
> 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)?

I was inspired by memset, but, in hindsight, it doesn't make sense for
clear_page to be weak.

>
> Just followed up with a patch to fix the modpost.

Thanks!

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

Great news!

Thanks,
drew