Re: [PATCH v4 5/5] arm64: mte: Inline mte_assign_mem_tag_range()

[Catalin Marinas]

On Mon, Jan 18, 2021 at 06:30:33PM +0000, Vincenzo Frascino wrote:
> mte_assign_mem_tag_range() is called on production KASAN HW hot
> paths. It makes sense to inline it in an attempt to reduce the
> overhead.
> 
> Inline mte_assign_mem_tag_range() based on the indications provided at
> [1].
> 
> [1] https://lore.kernel.org/r/CAAeHK+wCO+J7D1_T89DG+jJrPLk3X9RsGFKxJGd0ZcUFjQT-9Q@xxxxxxxxxxxxxx/
> 
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> ---
>  arch/arm64/include/asm/mte.h | 26 +++++++++++++++++++++++++-
>  arch/arm64/lib/mte.S         | 15 ---------------
>  2 files changed, 25 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
> index 237bb2f7309d..1a6fd53f82c3 100644
> --- a/arch/arm64/include/asm/mte.h
> +++ b/arch/arm64/include/asm/mte.h
> @@ -49,7 +49,31 @@ long get_mte_ctrl(struct task_struct *task);
>  int mte_ptrace_copy_tags(struct task_struct *child, long request,
>  			 unsigned long addr, unsigned long data);
>  
> -void mte_assign_mem_tag_range(void *addr, size_t size);
> +static inline void mte_assign_mem_tag_range(void *addr, size_t size)
> +{
> +	u64 _addr = (u64)addr;
> +	u64 _end = _addr + size;
> +
> +	/*
> +	 * This function must be invoked from an MTE enabled context.
> +	 *
> +	 * Note: The address must be non-NULL and MTE_GRANULE_SIZE aligned and
> +	 * size must be non-zero and MTE_GRANULE_SIZE aligned.
> +	 */
> +	do {
> +		/*
> +		 * 'asm volatile' is required to prevent the compiler to move
> +		 * the statement outside of the loop.
> +		 */
> +		asm volatile(__MTE_PREAMBLE "stg %0, [%0]"
> +			     :
> +			     : "r" (_addr)
> +			     : "memory");
> +
> +		_addr += MTE_GRANULE_SIZE;
> +	} while (_addr != _end);
> +}

While I'm ok with moving this function to C, I don't think it solves the
inlining in the kasan code. The only interface we have to kasan is via
mte_{set,get}_mem_tag_range(), so the above function doesn't need to
live in a header.

If you do want inlining all the way to the kasan code, we should
probably move the mte_{set,get}_mem_tag_range() functions to the header
as well (and ideally backed by some numbers to show that it matters).

Moving it to mte.c also gives us more control on how it's called (we
have the WARN_ONs in place in the callers).

-- 
Catalin