Re: [PATCH v4 6/8] RISC-V: Use Zicboz in clear_page when available

Andrew Jones
2023-02-10T08:06:29Z

On Thu, Feb 09, 2023 at 07:09:53PM +0000, Conor Dooley wrote:
> On Thu, Feb 09, 2023 at 04:26:26PM +0100, 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 taking advantage of 'vendor_id'
> > being used as application-specific data for alternatives, enabling 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>
>
> > diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> > index 74736b4f0624..42246bbfa532 100644
> > --- a/arch/riscv/kernel/cpufeature.c
> > +++ b/arch/riscv/kernel/cpufeature.c
> > @@ -280,6 +280,17 @@ void __init riscv_fill_hwcap(void)
> > #ifdef CONFIG_RISCV_ALTERNATIVE
> > static bool riscv_cpufeature_application_check(u32 feature, u16 data)
> > {
> > + switch (feature) {
> > + case RISCV_ISA_EXT_ZICBOZ:
> > + /*
> > + * Zicboz alternative applications provide the maximum
>
> I like the comment, rather than this being some wizardry.
> I find the word "applications" to be a little unclear, perhaps, iff this
> series needs a respin, this would work better as "Users of the Zicboz
> alternative provide..." (or s/Users/Callers)?

Right, "applications" is an overloaded word. "users" is probably a better
choice. "callers" isn't quite right, to me, since it's a code patching
"application" / "use". Do you think the function name should change as
well?

Thanks,
drew

>
> > + * 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 data == 0 || riscv_cboz_block_size <= (1U << data);
> > + }
> > +
> > return data == 0;
> > }