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

[alvise rigo]

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.

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