On 5/16/23 06:29, Peter Maydell wrote:
On Mon, 15 May 2023 at 15:38, Richard Henderson <richard.hender...@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.hender...@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. r~
