linux-kernel - Re: [v2 PATCH] arm64: mm: force write fault for atomic RMW instructions

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Message-ID: <Zk-SNVyEHT1UsxqD@arm.com>
Date: Thu, 23 May 2024 20:00:05 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: "Christoph Lameter (Ampere)" <cl@...two.org>
Cc: Yang Shi <yang@...amperecomputing.com>, will@...nel.org,
	anshuman.khandual@....com, scott@...amperecomputing.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH] arm64: mm: force write fault for atomic RMW
 instructions

On Thu, May 23, 2024 at 11:09:11AM -0700, Christoph Lameter (Ampere) wrote:
> On Thu, 23 May 2024, Catalin Marinas wrote:
> 
> > On Mon, May 20, 2024 at 09:56:36AM -0700, Yang Shi wrote:
> > > diff --git a/arch/arm64/include/asm/insn.h b/arch/arm64/include/asm/insn.h
> > > index db1aeacd4cd9..1cc73664fc55 100644
> > > --- a/arch/arm64/include/asm/insn.h
> > > +++ b/arch/arm64/include/asm/insn.h
> > > @@ -319,6 +319,7 @@ static __always_inline u32 aarch64_insn_get_##abbr##_value(void)	\
> > >   * "-" means "don't care"
> > >   */
> > >  __AARCH64_INSN_FUNCS(class_branch_sys,	0x1c000000, 0x14000000)
> > > +__AARCH64_INSN_FUNCS(class_atomic,	0x3b200c00, 0x38200000)
> > 
> > While this class includes all atomics that currently require write
> > permission, there's some unallocated space in this range and we don't
> > know what future architecture versions may introduce. Unfortunately we
> > need to check each individual atomic op in this class (not sure what the
> > overhead will be).
> 
> Can you tell us which bits or pattern is not allocated? Maybe we can exclude
> that from the pattern.

Yes, it may be easier to exclude those patterns. See the Arm ARM K.a
section C4.1.94.29 (page 791).

-- 
Catalin