linux-kernel - Re: [PATCH v3] riscv: Fix the PAUSE Opcode for MIPS P8700.

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Message-ID: <20250131-unknotted-yoga-bccacad0c6d3@spud>
Date: Fri, 31 Jan 2025 00:43:12 +0000
From: Conor Dooley <conor@...nel.org>
To: Charlie Jenkins <charlie@...osinc.com>
Cc: Aleksandar Rikalo <arikalo@...il.com>, linux-riscv@...ts.infradead.org,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Andrew Jones <ajones@...tanamicro.com>,
	Christoph Müllner <christoph.muellner@...ll.eu>,
	linux-kernel@...r.kernel.org,
	Djordje Todorovic <djordje.todorovic@...cgroup.com>
Subject: Re: [PATCH v3] riscv: Fix the PAUSE Opcode for MIPS P8700.

On Thu, Jan 30, 2025 at 02:58:49PM -0800, Charlie Jenkins wrote:
> On Wed, Jan 29, 2025 at 04:19:58PM +0000, Conor Dooley wrote:
> > On Wed, Jan 29, 2025 at 02:17:03PM +0100, Aleksandar Rikalo wrote:
> > > From: Djordje Todorovic <djordje.todorovic@...cgroup.com>
> > > 
> > > The riscv MIPS P8700 uses a different opcode for PAUSE.
> > > It is a ‘hint’ encoding of the SLLI instruction, with rd=0, rs1=0 and
> > > imm=5. It will behave as a NOP instruction if no additional behavior
> > > beyond that of SLLI is implemented.
> > 
> > You say p8700, but the erratum will be applied on all systems that are
> > identified as using a mips cpu. Why's that?
> > 
> > > +void mips_errata_patch_func(struct alt_entry *begin,
> > > +					     struct alt_entry *end,
> > > +					     unsigned long archid,
> > > +					     unsigned long impid,
> > > +					     unsigned int stage)
> > > +{
> > > +	struct alt_entry *alt;
> > > +
> > > +	BUILD_BUG_ON(ERRATA_MIPS_NUMBER >= RISCV_VENDOR_EXT_ALTERNATIVES_BASE);
> > > +
> > > +	if (stage == RISCV_ALTERNATIVES_EARLY_BOOT)
> > > +		return;
> > > +
> > > +	for (alt = begin; alt < end; alt++) {
> > > +		if (alt->vendor_id != MIPS_VENDOR_ID)
> > > +			continue;
> > > +
> > > +		if (alt->patch_id >= ERRATA_MIPS_NUMBER) {
> > > +			WARN(1, "MIPS errata id:%d not in kernel errata list\n",
> > > +			     alt->patch_id);
> > > +			continue;
> > > +		}
> > > +
> > > +		mutex_lock(&text_mutex);
> > > +		patch_text_nosync(ALT_OLD_PTR(alt), ALT_ALT_PTR(alt), alt->alt_len);
> > > +		mutex_unlock(&text_mutex);
> > > +	}
> > > +}
> > 
> > > diff --git a/tools/arch/riscv/include/asm/vdso/processor.h b/tools/arch/riscv/include/asm/vdso/processor.h
> > > index 662aca039848..880f26a24f69 100644
> > > --- a/tools/arch/riscv/include/asm/vdso/processor.h
> > > +++ b/tools/arch/riscv/include/asm/vdso/processor.h
> > > @@ -14,7 +14,10 @@ static inline void cpu_relax(void)
> > >  	__asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
> > >  #endif
> > >  
> > > -#ifdef CONFIG_TOOLCHAIN_HAS_ZIHINTPAUSE
> > > +#ifdef CONFIG_ERRATA_MIPS_P8700_PAUSE_OPCODE
> > > +    /* MIPS P8700 pause opcode */
> > > +    __asm__ __volatile__ (".4byte 0x00501013");
> > > +#elif CONFIG_TOOLCHAIN_HAS_ZIHINTPAUSE
> > >  	/*
> > >  	 * Reduce instruction retirement.
> > >  	 * This assumes the PC changes.
> > 
> > What about when the erratum is enabled and the toolchain supports
> > Zihintpause?
> 
> So the other way to do this is having an hwprobe call to check if the
> currently running processor is effected by this. However I was concerned
> about the performance penalty of calling hwprobe here in the previous
> version so I had suggested to use a flag instead so there is not the
> penalty on other architectures. This does make it invalid to enable
> this errata in the defconfig. This is a precedent for how we want to
> handle errata in tools.

Gonna have to be marked non-portable then, if enabling it results in
tools that might malfunction on other platforms.

> > Why don't you use the same implementation as the !tools
> > copy of the header? (I'm not sure why they're different in the first
> > place).
> 
> It is different because the headers in tools are userspace so it doesn't
> make sense to have alternatives.

I assume that's an answer to the first part, and not the bit in
brackets since the "first place" difference is about using the .4byte
versus having an ifdef/else and you're talking about alternatives.
That looks to have been some sort of sync issue or oversight in
6da111574baff, no?

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