linux-kernel - Re: [PATCH v3 4/8] RISC-V: Check Zicclsm to set unaligned access speed

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Date: Thu, 27 Jun 2024 14:20:44 -0700
From: Charlie Jenkins <charlie@...osinc.com>
To: Conor Dooley <conor@...nel.org>
Cc: Jesse Taube <jesse@...osinc.com>, linux-riscv@...ts.infradead.org,
	Jonathan Corbet <corbet@....net>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Clément Léger <cleger@...osinc.com>,
	Evan Green <evan@...osinc.com>,
	Andrew Jones <ajones@...tanamicro.com>,
	Xiao Wang <xiao.w.wang@...el.com>, Andy Chiu <andy.chiu@...ive.com>,
	Eric Biggers <ebiggers@...gle.com>,
	Greentime Hu <greentime.hu@...ive.com>,
	Björn Töpel <bjorn@...osinc.com>,
	Heiko Stuebner <heiko@...ech.de>,
	Costa Shulyupin <costa.shul@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Baoquan He <bhe@...hat.com>, Anup Patel <apatel@...tanamicro.com>,
	Zong Li <zong.li@...ive.com>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Ben Dooks <ben.dooks@...ethink.co.uk>,
	Alexandre Ghiti <alexghiti@...osinc.com>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Erick Archer <erick.archer@....com>,
	Joel Granados <j.granados@...sung.com>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH v3 4/8] RISC-V: Check Zicclsm to set unaligned access
 speed

On Wed, Jun 26, 2024 at 03:39:14PM +0100, Conor Dooley wrote:
> On Mon, Jun 24, 2024 at 08:49:57PM -0400, Jesse Taube wrote:
> > Check for Zicclsm before checking for unaligned access speed. This will
> > greatly reduce the boot up time as finding the access speed is no longer
> > necessary.
> > 
> > Signed-off-by: Jesse Taube <jesse@...osinc.com>
> > ---
> > V2 -> V3:
> >  - New patch split from previous patch
> > ---
> >  arch/riscv/kernel/unaligned_access_speed.c | 26 ++++++++++++++--------
> >  1 file changed, 17 insertions(+), 9 deletions(-)
> > 
> > diff --git a/arch/riscv/kernel/unaligned_access_speed.c b/arch/riscv/kernel/unaligned_access_speed.c
> > index a9a6bcb02acf..329fd289b5c8 100644
> > --- a/arch/riscv/kernel/unaligned_access_speed.c
> > +++ b/arch/riscv/kernel/unaligned_access_speed.c
> > @@ -259,23 +259,31 @@ static int check_unaligned_access_speed_all_cpus(void)
> >  	kfree(bufs);
> >  	return 0;
> >  }
> > +#else /* CONFIG_RISCV_PROBE_UNALIGNED_ACCESS */
> > +static int check_unaligned_access_speed_all_cpus(void)
> > +{
> > +	return 0;
> > +}
> > +#endif
> >  
> >  static int check_unaligned_access_all_cpus(void)
> >  {
> > -	bool all_cpus_emulated = check_unaligned_access_emulated_all_cpus();
> > +	bool all_cpus_emulated;
> > +	int cpu;
> > +
> > +	if (riscv_has_extension_unlikely(RISCV_ISA_EXT_ZICCLSM)) {
> > +		for_each_online_cpu(cpu) {
> > +			per_cpu(misaligned_access_speed, cpu) = RISCV_HWPROBE_MISALIGNED_FAST;
> 
> - const: zicclsm
>   description:
>     The standard Zicclsm extension for misaligned support for all regular
>     load and store instructions (including scalar and vector) but not AMOs
>     or other specialized forms of memory access. Defined in the
>     RISC-V RVA Profiles Specification. 
> 
> Doesn't, unfortunately, say anywhere there that they're actually fast :(

Oh no! That is unfortunate that the ISA does not explicitly call that
out, but I think that acceptable.

If a vendor puts Zicclsm in their isa string, they should expect
software to take advantage of misaligned accesses. FAST is our signal to
tell software that they should emit misaligned accesses.

This allows for a generic kernel, like the one a distro would compile, to
skip the probing when booting on a system that explicitly called out
that the hardware supports misaligned accesses.

- Charlie

> 
> Thanks,
> Conor.
> 
> > +		}
> > +		return 0;
> > +	}
> > +
> > +	all_cpus_emulated = check_unaligned_access_emulated_all_cpus();
> >  
> >  	if (!all_cpus_emulated)
> >  		return check_unaligned_access_speed_all_cpus();
> >  
> >  	return 0;
> >  }
> > -#else /* CONFIG_RISCV_PROBE_UNALIGNED_ACCESS */
> > -static int check_unaligned_access_all_cpus(void)
> > -{
> > -	check_unaligned_access_emulated_all_cpus();
> > -
> > -	return 0;
> > -}
> > -#endif
> >  
> >  arch_initcall(check_unaligned_access_all_cpus);
> > -- 
> > 2.45.2
> >