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

Conor Dooley
2023-02-10T09:06:32Z

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On Fri, Feb 10, 2023 at 09:05:15AM +0100, Andrew Jones wrote:
> 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?

I was initially going to suggest that too, but then couldn't really
think of something better. s/application_check/check_applies/ maybe?

> > > + * 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;
> > > }

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