Re: [Xen-devel] [RFC PATCH] arm64: make bitops 32-bit tolerant

[Will Deacon <will.deacon@xxxxxxx>]

On Sun, Apr 27, 2014 at 10:21:17AM +0100, Vladimir Murzin wrote:
> Xen assumes that bit operations are able to operate on 32-bit size and
> alignment [1]. For arm64 bitops are based on atomic exclusive load/store
> instructions to guarantee that changes are made atomically. However, these
> instructions require that address to be aligned to the data size. Because, by
> default, bitops operates on 64-bit size it implies that address should be
> aligned appropriately. All these lead to breakage of Xen assumption for bitops
> properties.
> 
> This patch turns bitops to operate on 32-bit size/alignment.

Please stop sending this patch. I have _absolutely_ _no_ intention of
butchering our bitops in this way, and I don't think the Xen guys require
changing the core bitops like this either (certainly, Ian's last reply to
you suggested you should be doing this another way).

NAK.

Will

> [1] http://www.gossamer-threads.com/lists/xen/devel/325613
> 
> Cc: catalin.marinas@xxxxxxx
> Cc: will.deacon@xxxxxxx
> Cc: david.vrabel@xxxxxxxxxx
> Cc: Ian.Campbell@xxxxxxxxxx
> 
> Signed-off-by: Vladimir Murzin <murzin.v@xxxxxxxxx>
> ---
>  With this patch applied I was able to boot both Dom0 and DomU.
>  
>  @Pranav: Could I have your Tested-by on this patch?
>  
>  arch/arm64/lib/bitops.S          | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/arm64/lib/bitops.S b/arch/arm64/lib/bitops.S
> index 7dac371..a6acfff 100644
> --- a/arch/arm64/lib/bitops.S
> +++ b/arch/arm64/lib/bitops.S
> @@ -26,14 +26,14 @@
>   */
>       .macro  bitop, name, instr
>  ENTRY(       \name   )
> -     and     w3, w0, #63             // Get bit offset
> +     and     w3, w0, #31             // Get bit offset
>       eor     w0, w0, w3              // Clear low bits
>       mov     x2, #1
>       add     x1, x1, x0, lsr #3      // Get word offset
>       lsl     x3, x2, x3              // Create mask
> -1:   ldxr    x2, [x1]
> -     \instr  x2, x2, x3
> -     stxr    w0, x2, [x1]
> +1:   ldxr    w2, [x1]
> +     \instr  w2, w2, w3
> +     stxr    w0, w2, [x1]
>       cbnz    w0, 1b
>       ret
>  ENDPROC(\name        )
> @@ -41,15 +41,15 @@ ENDPROC(\name     )
>  
>       .macro  testop, name, instr
>  ENTRY(       \name   )
> -     and     w3, w0, #63             // Get bit offset
> +     and     w3, w0, #31             // Get bit offset
>       eor     w0, w0, w3              // Clear low bits
>       mov     x2, #1
>       add     x1, x1, x0, lsr #3      // Get word offset
>       lsl     x4, x2, x3              // Create mask
> -1:   ldxr    x2, [x1]
> +1:   ldxr    w2, [x1]
>       lsr     x0, x2, x3              // Save old value of bit
> -     \instr  x2, x2, x4              // toggle bit
> -     stlxr   w5, x2, [x1]
> +     \instr  w2, w2, w4              // toggle bit
> +     stlxr   w5, w2, [x1]
>       cbnz    w5, 1b
>       dmb     ish
>       and     x0, x0, #1
> -- 
> 1.8.3.2
> 
> 

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