linux-kernel - Re: [PATCH 2/3] MIPS: Add barrier between dcache & icache flushes

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Message-ID: <20171221145801.GF5027@jhogan-linux.mipstec.com>
Date:   Thu, 21 Dec 2017 14:58:02 +0000
From:   James Hogan <james.hogan@...s.com>
To:     Matt Redfearn <matt.redfearn@...s.com>
CC:     Ralf Baechle <ralf@...ux-mips.org>, <linux-mips@...ux-mips.org>,
        "stable # v4 . 9+" <stable@...r.kernel.org>,
        Huacai Chen <chenhc@...ote.com>,
        <linux-kernel@...r.kernel.org>, Paul Burton <paul.burton@...s.com>
Subject: Re: [PATCH 2/3] MIPS: Add barrier between dcache & icache flushes

On Thu, Dec 21, 2017 at 11:16:03AM +0000, Matt Redfearn wrote:
> Index-based cache operations may be arbitrarily reordered by out of
> order CPUs. Thus code which writes back the dcache & then invalidates
> the icache using indexed cache ops must include a barrier between
> operating on the 2 caches in order to prevent the scenario in which:
> 
>   - icache invalidation occurs.
>   - icache fetch occurs, due to speculation.
>   - dcache writeback occurs.
> 
> If the above were allowed to happen then the icache would contain stale
> data. Forcing the dcache writeback to complete before the icache
> invalidation avoids this.
> 
> Similarly, the MIPS CM version 2 and above serialises D->I hit-based
> cache operations to the same address, but older CMs and systems without
> a MIPS CM do not and require the same barrier to ensure ordering.
> 
> To ensure these conditions, always enforce a barrier between D and I
> cache operations.
> 
> Suggested-by: Leonid Yegoshin <Leonid.Yegoshin@...s.com>
> Suggested-by: Paul Burton <paul.burton@...s.com>
> Signed-off-by: Matt Redfearn <matt.redfearn@...s.com>
> Cc: James Hogan <james.hogan@...s.com>
> Cc: stable <stable@...r.kernel.org> # v4.9+

Looks reasonable to me,

Reviewed-by: James Hogan <jhogan@...nel.org>

Cheers
James

> ---
> 
>  arch/mips/mm/c-r4k.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/mips/mm/c-r4k.c b/arch/mips/mm/c-r4k.c
> index ce7a54223504..b7186d47184b 100644
> --- a/arch/mips/mm/c-r4k.c
> +++ b/arch/mips/mm/c-r4k.c
> @@ -741,6 +741,9 @@ static inline void __local_r4k_flush_icache_range(unsigned long start,
>  			else
>  				blast_dcache_range(start, end);
>  		}
> +
> +		/* Ensure dcache operation has completed */
> +		mb();
>  	}
>  
>  	if (type == R4K_INDEX ||
> -- 
> 2.7.4
>