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

From: Vincenzo Frascino <vincenzo.frascino@arm.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com,
	Will Deacon <will@kernel.org>, Dmitry Vyukov <dvyukov@google.com>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Alexander Potapenko <glider@google.com>,
	Marco Elver <elver@google.com>,
	Evgenii Stepanov <eugenis@google.com>,
	Branislav Rankov <Branislav.Rankov@arm.com>,
	Andrey Konovalov <andreyknvl@google.com>
Subject: Re: [PATCH v4 5/5] arm64: mte: Inline mte_assign_mem_tag_range()
Date: Tue, 19 Jan 2021 15:48:56 +0000	[thread overview]
Message-ID: <1bb4355f-4341-21a7-0a53-a4a27840adee@arm.com> (raw)
In-Reply-To: <20210119144459.GE17369@gaia>

Hi Catalin,

On 1/19/21 2:45 PM, Catalin Marinas wrote:
> 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@mail.gmail.com/
>>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Will Deacon <will@kernel.org>
>> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
>> ---
>>  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).
> 

Based on the thread [1] this patch contains only an intermediate step to allow
KASAN to call directly mte_assign_mem_tag_range() in future. At that point I
think that mte_set_mem_tag_range() can be removed.

If you agree, I would live the things like this to give to Andrey a chance to
execute on the original plan with a separate series.

I agree though that this change alone does not bring huge benefits but
regressions neither.

If you want I can add something to the commit message in the next version to
make this more explicit.

Let me know how do you want me to proceed.

-- 
Regards,
Vincenzo