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

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Jan 19, 2021 at 07:12:40PM +0100, Andrey Konovalov wrote:
> On Tue, Jan 19, 2021 at 4:45 PM Vincenzo Frascino
> <vincenzo.frascino@arm.com> wrote:
> > 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 think we should drop this patch from this series as it's unrelated.
> 
> I will pick it up into my future optimization series. Then it will be
> easier to discuss it in the context. The important part that I needed
> is an inlinable C implementation of mte_assign_mem_tag_range(), which
> I now have with this patch.

That's fine by me but we may want to add some forced-alignment on the
addr and size as the loop here depends on them being aligned, otherwise
it gets stuck. The mte_set_mem_tag_range() at least had a WARN_ON in
place. Here we could do:

	addr &= MTE_GRANULE_MASK;
	size = ALIGN(size, MTE_GRANULE_SIZE);

(or maybe trim "size" with MTE_GRANULE_MASK)

That's unless the call places are well known and guarantee this
alignment (only had a very brief look).

-- 
Catalin

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