linux-kernel - Re: [PATCH 2/2] ARM: domains: add memory dependencies to get_domain/set

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Message-ID: <87si6lt36s.fsf@belgarion.home>
Date:	Fri, 11 Sep 2015 16:56:27 +0200
From:	Robert Jarzmik <robert.jarzmik@...e.fr>
To:	Russell King <rmk+kernel@....linux.org.uk>
Cc:	Dave Martin <Dave.Martin@....com>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/2] ARM: domains: add memory dependencies to get_domain/set_domain

Russell King <rmk+kernel@....linux.org.uk> writes:

> We need to have memory dependencies on get_domain/set_domain to avoid
> the compiler over-optimising these inline assembly instructions.
>
> Loads/stores must not be reordered across a set_domain(), so introduce
> a compiler barrier for that assembly.
>
> The value of get_domain() must not be cached across a set_domain(), but
> we still want to allow the compiler to optimise it away.  Introduce a
> dependency on current_thread_info()->cpu_domain to avoid this; the new
> memory clobber in set_domain() should therefore cause the compiler to
> re-load this.  The other advantage of using this is we should have its
> address in the register set already, or very soon after at most call
> sites.
>
> Signed-off-by: Russell King <rmk+kernel@....linux.org.uk>
Tested-by: Robert Jarzmik <robert.jarzmik@...e.fr>

The test is failing without these 2 patches, while with them, an unaligned
access is fixed, and the generated code looks good :
     7ac:       ee13cf10        mrc     15, 0, ip, cr3, cr0, {0}
     7b0:       e3cc300c        bic     r3, ip, #12
     7b4:       e58dc014        str     ip, [sp, #20]
     7b8:       e3833004        orr     r3, r3, #4
     7bc:       ee033f10        mcr     15, 0, r3, cr3, cr0, {0}

Cheers.

--
Robert
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