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

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:

quoted

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/ (local)

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

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