Re: [PATCH v5 6/8] string: introduce memcpy_nt() helpers

[David Hildenbrand (Arm)]

On 7/1/26 11:05, Li Zhe wrote:
> Introduce memcpy_nt() and memcpy_nt_drain() for write-once copy sites
> that want a named non-temporal copy primitive plus an explicit ordering
> point. On x86, place the arch-visible wrapper in
> arch/x86/include/asm/string_64.h and map it to the existing
> memcpy_flushcache() backend plus sfence. Architectures that do not
> override the helper fall back to memcpy() and a no-op drain in
> include/linux/string.h.
> 
> The immediate user is the ZONE_DEVICE template-copy path. That path
> populates struct page descriptors in a write-once pattern, so most
> destination cachelines are not expected to be reused immediately after
> the copy. A regular cached memcpy() can therefore incur avoidable
> write-allocate traffic and pollute the cache with data that has little
> near-term reuse.
> 
> This interface lets callers request that non-temporal-copy semantics
> directly, while x86 simply reuses the existing memcpy_flushcache()
> backend instead of adding another generic memcpy-like wrapper with
> extra selection policy above it.
> 
> Signed-off-by: Li Zhe <lizhe.67@xxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/string_64.h | 16 ++++++++++++++++
>  include/linux/string.h           | 18 ++++++++++++++++++
>  2 files changed, 34 insertions(+)
> 
> diff --git a/arch/x86/include/asm/string_64.h b/arch/x86/include/asm/string_64.h
> index 4635616863f5..6f36abedc56a 100644
> --- a/arch/x86/include/asm/string_64.h
> +++ b/arch/x86/include/asm/string_64.h
> @@ -100,6 +100,22 @@ static __always_inline void memcpy_flushcache(void *dst, const void *src, size_t
>  	}
>  	__memcpy_flushcache(dst, src, cnt);
>  }
> +
> +#define __HAVE_ARCH_MEMCPY_NT 1
> +/*
> + * Reuse the existing x86 flushcache backend as the nt copy primitive.
> + * Callers pair it with memcpy_nt_drain() when later stores must be
> + * ordered after the copy.
> + */
> +static __always_inline void memcpy_nt(void *dst, const void *src, size_t cnt)
> +{
> +	memcpy_flushcache(dst, src, cnt);
> +}

In particular if you end up with a single function (that jsut requires a write
memory barrier afterwards), please make this

#define memcpy_nt memcpy_nt

instead.

-- 
Cheers,

David