Re: [PATCH v5 11/54] accel/tcg: Add aarch64 specific support in ldst_atomicity

[Peter Maydell <peter.maydell@linaro.org>]

On Tue, 16 May 2023 at 14:51, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> On 5/16/23 06:29, Peter Maydell wrote:
> > On Mon, 15 May 2023 at 15:38, Richard Henderson
> > <richard.henderson@linaro.org> wrote:
> >>
> >> We have code in atomic128.h noting that through GCC 8, there
> >> was no support for atomic operations on __uint128.  This has
> >> been fixed in GCC 10.  But we can still improve over any
> >> basic compare-and-swap loop using the ldxp/stxp instructions.
> >>
> >> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> >> ---
> >>   accel/tcg/ldst_atomicity.c.inc | 60 ++++++++++++++++++++++++++++++++--
> >>   1 file changed, 57 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/accel/tcg/ldst_atomicity.c.inc b/accel/tcg/ldst_atomicity.c.inc
> >> index 69c1c61997..c3b2b35823 100644
> >> --- a/accel/tcg/ldst_atomicity.c.inc
> >> +++ b/accel/tcg/ldst_atomicity.c.inc
> >> @@ -263,7 +263,22 @@ static Int128 load_atomic16_or_exit(CPUArchState *env, uintptr_t ra, void *pv)
> >>        * In system mode all guest pages are writable, and for user-only
> >>        * we have just checked writability.  Try cmpxchg.
> >>        */
> >> -#if defined(CONFIG_CMPXCHG128)
> >> +#if defined(__aarch64__)
> >> +    /* We can do better than cmpxchg for AArch64.  */
> >> +    {
> >> +        uint64_t l, h;
> >> +        uint32_t fail;
> >> +
> >> +        /* The load must be paired with the store to guarantee not tearing. */
> >> +        asm("0: ldxp %0, %1, %3\n\t"
> >> +            "stxp %w2, %0, %1, %3\n\t"
> >> +            "cbnz %w2, 0b"
> >> +            : "=&r"(l), "=&r"(h), "=&r"(fail) : "Q"(*p));
> >> +
> >> +        qemu_build_assert(!HOST_BIG_ENDIAN);
> >> +        return int128_make128(l, h);
> >> +    }
> >
> > The compiler (well, clang 11, anyway) seems able to generate equivalent
> > code to this inline asm:
>
> See above, where GCC 8 can do nothing, and that is still a supported compiler.

Yeah, but it'll work fine even without the explicit inline
asm, right? It just might generate slightly worse code.
Is the performance difference critical enough to justify
an inline asm implementation that's only needed for older
compilers ?

thanks
-- PMM