Re: [Qemu-devel] [PATCH v2 09/12] cpus: introduce cpu_update_icount helper

[Paolo Bonzini]

On 05/04/2017 16:34, Alex Bennée wrote:
>> On 05/04/2017 15:25, Alex Bennée wrote:
>>> +{
>>> +    int64_t executed = cpu_get_icount_executed(cpu);
>>> +    cpu->icount_budget -= executed;
>>> +    atomic_add(&timers_state.qemu_icount, executed);
>>> +}
>> Since there's only one writer, it's also okay to do
>>
>>      atomic_set(&timers_state.qemu_icount,
>>                 atomic_read(&timers_state.qemu_icount) + executed);
>>
>> (also not just faster, but also simpler to turn into TCG code if we do
>> the update in gen_io_start).
>
> OK fair enough. Annoyingly I've just noticed this breaks for 32 bit
> hosts because we might not have CONFIG_ATOMIC64. What's the best
> approach? #ifdef and handwave?

For 2.9 yes... For 2.10 I'm going to add a "Stat64" type that can
compute max or add of 63-bit values, and blocks on the read side if a
doubleword change is in progress.

Paolo