Re: [Qemu-devel] [RFC 01/10] exec: Introduce tcg_exclusive_{lock, unlock}()

[Alex Bennée]

alvise rigo <address@hidden> writes:

> As far as I understand, linux-user uses a mutex to make the atomic
> accesses exclusive with respect to other CPU's atomic accesses. So
> basically in the LDREX case it implements:
> lock() -> access() -> unlock()
> This patch series makes the atomic accesses exclusive with respect to
> every memory access, this is allowed by the softmmu. The access is now
> something like:
> lock() -> softmmu_access() -> unlock()
> where "softmmu_access()" is not just a memory access, but includes a
> manipulation of the EXCL bitmap and possible queries of TLB flushes.
> So there are similarities, but are pretty much confined to the
> locking/unlocking of a spinlock/mutex.

But couldn't this be achieved with the various other solutions to
wanting the rest of the system to be quiescent while we do something
tricky?

> This made me think, how does linux-user can properly work with
> upstream TCG, for instance, in an absurd configuration like target-arm
> on ARM host?
>
> alvise
>
> On Wed, Jun 8, 2016 at 11:21 AM, Alex Bennée <address@hidden> wrote:
>>
>> Alvise Rigo <address@hidden> writes:
>>
>>> Add tcg_exclusive_{lock,unlock}() functions that will be used for making
>>> the emulation of LL and SC instructions thread safe.
>>
>> I wonder how much similarity there is to the mechanism linus-user ends
>> up using for it's exclusive-start/end?
>>
>>>
>>> Signed-off-by: Alvise Rigo <address@hidden>
>>> ---
>>>  cpus.c            |  2 ++
>>>  exec.c            | 18 ++++++++++++++++++
>>>  include/qom/cpu.h |  5 +++++
>>>  3 files changed, 25 insertions(+)
>>>
>>> diff --git a/cpus.c b/cpus.c
>>> index 860e7ba..b9ec903 100644
>>> --- a/cpus.c
>>> +++ b/cpus.c
>>> @@ -961,6 +961,8 @@ void qemu_init_cpu_loop(void)
>>>      qemu_cond_init(&qemu_work_cond);
>>>      qemu_mutex_init(&qemu_global_mutex);
>>>
>>> +    qemu_spin_init(&cpu_exclusive_lock);
>>> +
>>>      qemu_thread_get_self(&io_thread);
>>>
>>>      safe_work = g_array_sized_new(TRUE, TRUE, sizeof(qemu_safe_work_item), 
>>> 128);
>>> diff --git a/exec.c b/exec.c
>>> index a24b31c..1c72113 100644
>>> --- a/exec.c
>>> +++ b/exec.c
>>> @@ -197,6 +197,24 @@ void cpu_exclusive_history_free(void)
>>>          g_free(excl_history.c_array);
>>>      }
>>>  }
>>> +
>>> +__thread bool cpu_have_exclusive_lock;
>>> +QemuSpin cpu_exclusive_lock;
>>> +inline void tcg_exclusive_lock(void)
>>> +{
>>> +    if (!cpu_have_exclusive_lock) {
>>> +        qemu_spin_lock(&cpu_exclusive_lock);
>>> +        cpu_have_exclusive_lock = true;
>>> +    }
>>> +}
>>> +
>>> +inline void tcg_exclusive_unlock(void)
>>> +{
>>> +    if (cpu_have_exclusive_lock) {
>>> +        cpu_have_exclusive_lock = false;
>>> +        qemu_spin_unlock(&cpu_exclusive_lock);
>>> +    }
>>> +}
>>>  #endif
>>>
>>>  #if !defined(CONFIG_USER_ONLY)
>>> diff --git a/include/qom/cpu.h b/include/qom/cpu.h
>>> index 0f51870..019f06d 100644
>>> --- a/include/qom/cpu.h
>>> +++ b/include/qom/cpu.h
>>> @@ -201,6 +201,11 @@ typedef struct CPUClass {
>>>      void (*disas_set_info)(CPUState *cpu, disassemble_info *info);
>>>  } CPUClass;
>>>
>>> +/* Protect cpu_exclusive_* variable .*/
>>> +void tcg_exclusive_lock(void);
>>> +void tcg_exclusive_unlock(void);
>>> +extern QemuSpin cpu_exclusive_lock;
>>> +
>>>  #ifdef HOST_WORDS_BIGENDIAN
>>>  typedef struct icount_decr_u16 {
>>>      uint16_t high;
>>
>>
>> --
>> Alex Bennée


--
Alex Bennée