Re: [Qemu-arm] [PATCH 4/9] tcg: Introduce atomic helpers for integer min/max

[Peter Maydell]

On 3 May 2018 at 18:13, Richard Henderson <address@hidden> wrote:
> On 05/03/2018 06:26 AM, Peter Maydell wrote:
>> On 27 April 2018 at 01:26, Richard Henderson
>> <address@hidden> wrote:
>>> Given that this atomic operation will be used by both risc-v
>>> and aarch64, let's not duplicate code across the two targets.
>>>
>>> Signed-off-by: Richard Henderson <address@hidden>
>>> ---
>>>  accel/tcg/atomic_template.h | 71 
>>> +++++++++++++++++++++++++++++++++++++++++++++
>>>  accel/tcg/tcg-runtime.h     |  8 +++++
>>>  tcg/tcg-op.h                | 34 ++++++++++++++++++++++
>>>  tcg/tcg.h                   |  8 +++++
>>>  tcg/tcg-op.c                |  8 +++++
>>>  5 files changed, 129 insertions(+)
>>
>>> @@ -233,6 +270,39 @@ ABI_TYPE ATOMIC_NAME(add_fetch)(CPUArchState *env, 
>>> target_ulong addr,
>>>          ldo = ldn;
>>>      }
>>>  }
>>> +
>>> +/* These helpers are, as a whole, full barriers.  Within the helper,
>>> + * the leading barrier is explicit and the trailing barrier is within
>>> + * cmpxchg primitive.
>>> + */
>>> +#define GEN_ATOMIC_HELPER_FN(X, FN, XDATA_TYPE, RET)                \
>>> +ABI_TYPE ATOMIC_NAME(X)(CPUArchState *env, target_ulong addr,       \
>>> +                        ABI_TYPE xval EXTRA_ARGS)                   \
>>> +{                                                                   \
>>> +    ATOMIC_MMU_DECLS;                                               \
>>> +    XDATA_TYPE *haddr = ATOMIC_MMU_LOOKUP;                          \
>>> +    XDATA_TYPE ldo, ldn, old, new, val = xval;                      \
>>> +    smp_mb();                                                       \
>>> +    ldn = atomic_read__nocheck(haddr);                              \
>>
>> I see you're using the __nocheck function here. How does this
>> work for the 32-bit host case where you don't necessarily have
>> a 64-bit atomic primitive?
>
> It won't be compiled for the 32-bit host.  Translation will not attempt to use
> this helper and will instead call exit_atomic.

OK. Can you point me at the code that handles min/max atomics in that case?

thanks
-- PMM