Re: [Qemu-devel] [PATCH 1/9] main-loop: use qemu_mutex_lock_iothread consistently

[Fam Zheng]

On Wed, 06/24 18:25, Paolo Bonzini wrote:
> The next patch will require the BQL to be always taken with
> qemu_mutex_lock_iothread(), while right now this isn't the case.
> 
> Outside TCG mode this is not a problem.  In TCG mode, we need to be
> careful and avoid the "prod out of compiled code" step if already
> in a VCPU thread.  This is easily done with a check on current_cpu,
> i.e. qemu_in_vcpu_thread().
> 
> Hopefully, multithreaded TCG will get rid of the whole logic to kick
> VCPUs whenever an I/O event occurs!
> 
> Cc: Frederic Konrad <address@hidden>
> Message-Id: <address@hidden>

Why is this "Message-Id:" included in the commit message if it's not final?

> Signed-off-by: Paolo Bonzini <address@hidden>
> ---
>  cpus.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/cpus.c b/cpus.c
> index b85fb5f..02cca5d 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -953,7 +953,7 @@ static void *qemu_kvm_cpu_thread_fn(void *arg)
>      CPUState *cpu = arg;
>      int r;
>  
> -    qemu_mutex_lock(&qemu_global_mutex);
> +    qemu_mutex_lock_iothread();
>      qemu_thread_get_self(cpu->thread);
>      cpu->thread_id = qemu_get_thread_id();
>      cpu->can_do_io = 1;
> @@ -1033,10 +1033,10 @@ static void *qemu_tcg_cpu_thread_fn(void *arg)
>  {
>      CPUState *cpu = arg;
>  
> +    qemu_mutex_lock_iothread();
>      qemu_tcg_init_cpu_signals();
>      qemu_thread_get_self(cpu->thread);
>  
> -    qemu_mutex_lock(&qemu_global_mutex);
>      CPU_FOREACH(cpu) {
>          cpu->thread_id = qemu_get_thread_id();
>          cpu->created = true;
> @@ -1148,7 +1148,11 @@ bool qemu_in_vcpu_thread(void)
>  void qemu_mutex_lock_iothread(void)
>  {
>      atomic_inc(&iothread_requesting_mutex);
> -    if (!tcg_enabled() || !first_cpu || !first_cpu->thread) {
> +    /* In the simple case there is no need to bump the VCPU thread out of
> +     * TCG code execution.
> +     */
> +    if (!tcg_enabled() || qemu_in_vcpu_thread() ||
> +        !first_cpu || !first_cpu->thread) {

This looks like a separate change from the above
"qemu_mutex_lock(&qemu_global_mutex)" conversion. Why do they belong to the
same patch?

Fam

>          qemu_mutex_lock(&qemu_global_mutex);
>          atomic_dec(&iothread_requesting_mutex);
>      } else {
> -- 
> 1.8.3.1
> 
>