Re: [Xen-devel] [PATCH 2/2] xen: arm: update arm32 assembly primitives to Linux v3.16-rc6

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 07/25/2014 05:13 PM, Ian Campbell wrote:
> On Fri, 2014-07-25 at 17:03 +0100, Ian Campbell wrote:
>> On Fri, 2014-07-25 at 16:48 +0100, Julien Grall wrote:
>>> On 07/25/2014 04:48 PM, Ian Campbell wrote:
>>>> On Fri, 2014-07-25 at 16:42 +0100, Julien Grall wrote:
>>>>> Hi Ian,
>>>>>
>>>>> On 07/25/2014 04:22 PM, Ian Campbell wrote:
>>>>>> bitops, cmpxchg, atomics: Import:
>>>>>>   c32ffce ARM: 7984/1: prefetch: add prefetchw invocations for barriered 
>>>>>> atomics
>>>>>
>>>>> Compare to Linux we don't have specific prefetch* helpers. We directly
>>>>> use the compiler builtin ones. Shouldn't we import the ARM specific
>>>>> helpers to gain in performance?
>>>>
>>>> My binaries are full of pld instructions where I think I would expect
>>>> them, so it seems like the compiler builtin ones are sufficient.
>>>>
>>>> I suspect the Linux define is there to cope with older compilers or
>>>> something.
>>>
>>> If so:
>>
>> The compiled output is very different if I use the arch specific
>> explicit variants. The explicit variant generates (lots) more pldw and
>> (somewhat) fewer pld. I've no idea what this means...
> 
> It's a bit more obvious for aarch64 where gcc 4.8 doesn't generate any
> prefetches at all via the builtins...
> 
> Here's what I've got in my tree. I've no idea if we should take some or
> all of it...

I don't think it will be harmful for ARMv7 to use specific prefetch*
helpers.

[..]

> +/*
> + * Prefetching support
> + */
> +#define ARCH_HAS_PREFETCH
> +static inline void prefetch(const void *ptr)
> +{
> +        asm volatile("prfm pldl1keep, %a0\n" : : "p" (ptr));
> +}
> +
> +#define ARCH_HAS_PREFETCHW
> +static inline void prefetchw(const void *ptr)
> +{
> +        asm volatile("prfm pstl1keep, %a0\n" : : "p" (ptr));
> +}
> +
> +#define ARCH_HAS_SPINLOCK_PREFETCH
> +static inline void spin_lock_prefetch(const void *x)
> +{
> +        prefetchw(x);
> +}

Looking to the code. spin_lock_prefetch is called in the tree. I'm not
sure we should keep this helper.

Regards,

-- 
Julien Grall

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