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From: Peter Zijlstra <peterz@infradead.org>
To: Andrew Murray <andrew.murray@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Will Deacon <will.deacon@arm.com>,
	linux-arm-kernel@lists.infradead.org, Ard.Biesheuvel@arm.com
Subject: Re: [PATCH v1 0/5] arm64: avoid out-of-line ll/sc atomics
Date: Fri, 17 May 2019 14:05:22 +0200
Message-ID: <20190517120522.GM2623@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20190517100802.GS8268@e119886-lin.cambridge.arm.com>

On Fri, May 17, 2019 at 11:08:03AM +0100, Andrew Murray wrote:

> I think the alternative solution (excuse the pun) that you are suggesting
> is to put the body of the ll/sc or LSE code in the ALTERNATIVE oldinstr/newinstr
> blocks (i.e. drop the fallback branches). However this still gives us some
> bloat (but less than my current solution) because we're still now inlining the
> larger fallback ll/sc whereas previously they were non-inline'd functions. We
> still end up with potentially unnecessary clobbers for LSE code with this
> Approach prior to this series:

> Approach using alternative without braces:
> 
>    LSE
>    LSE
>    NOP
>    NOP
> 
> or
> 
>    LL/SC <- inlined LL/SC and thus duplicated
>    LL/SC
>    LL/SC
>    LL/SC

Yes that. And if you worry about the extra clobber for LL/SC, you could
always stuck a few PUSH/POPs around the LL/SC block. Although I'm not
exactly sure where the x16,x17,x30 clobbers come from; then I look at
the LL/SC code, there aren't any hard-coded regs in there.

Also, the safe approach is to emit LL/SC as the default and only patch
in LSE when you know the machine supports them.

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  parent reply index

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-16 15:53 Andrew Murray
2019-05-16 15:53 ` [PATCH v1 1/5] jump_label: Don't warn on __exit jump entries Andrew Murray
2019-05-16 15:53 ` [PATCH v1 2/5] arm64: Use correct ll/sc atomic constraints Andrew Murray
2019-05-16 15:53 ` [PATCH v1 3/5] arm64: atomics: avoid out-of-line ll/sc atomics Andrew Murray
2019-05-16 15:53 ` [PATCH v1 4/5] arm64: avoid using hard-coded registers for LSE atomics Andrew Murray
2019-05-16 15:53 ` [PATCH v1 5/5] arm64: atomics: remove atomic_ll_sc compilation unit Andrew Murray
2019-05-17  7:24 ` [PATCH v1 0/5] arm64: avoid out-of-line ll/sc atomics Peter Zijlstra
2019-05-17 10:08   ` Andrew Murray
2019-05-17 10:29     ` Ard Biesheuvel
2019-05-22 10:45       ` Andrew Murray
2019-05-22 11:44         ` Ard Biesheuvel
2019-05-22 15:36           ` Andrew Murray
2019-05-17 12:05     ` Peter Zijlstra [this message]
2019-05-17 12:19       ` Ard Biesheuvel

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