Pranith Kumar <bobby.pr...@gmail.com> writes:
> The current method of executing atomic code in a guest uses
> cpu_exec_step_atomic() from the outermost loop. This causes an abort()
> when single stepping over atomic code since debug exception longjmp
> will point to the the setlongjmp in cpu_exec(). Another issue with
> this mechanism is that the flags which were set in atomic execution
> will be lost since we do not call cpu_exec_enter().
I should not the original patch (which is still in my tree so I guess I
should squash it) says:
The patch enables handling atomic code in the guest. This should be
preferably done in cpu_handle_exception(), but the current assumptions
regarding when we can execute atomic sections cause a deadlock.
> The following patch moves atomic exception handling to the exception
> handler where all these issues are taken care of. The change in
> start_exclusive() is necessary since now the cpu in atomic execution
> will have its running flag set, but we do not want to count it as
> pending.
>
> Thanks to Alex for helping me debug the issue.
>
> CC: Alex Bennée <alex.ben...@linaro.org>
> CC: Richard Henderson <r...@twiddle.net>
> CC: Paolo Bonzini <pbonz...@redhat.com>
> Signed-off-by: Pranith Kumar <bobby.pr...@gmail.com>
> ---
> cpu-exec.c | 2 ++
> cpus-common.c | 2 +-
> cpus.c | 4 ----
> 3 files changed, 3 insertions(+), 5 deletions(-)
>
> diff --git a/cpu-exec.c b/cpu-exec.c
> index b0ddada8c1..dceacfc5dd 100644
> --- a/cpu-exec.c
> +++ b/cpu-exec.c
> @@ -427,6 +427,8 @@ static inline bool cpu_handle_exception(CPUState *cpu,
> int *ret)
> *ret = cpu->exception_index;
> if (*ret == EXCP_DEBUG) {
> cpu_handle_debug_exception(cpu);
> + } else if (*ret == EXCP_ATOMIC) {
> + cpu_exec_step_atomic(cpu);
> }
> cpu->exception_index = -1;
> return true;
> diff --git a/cpus-common.c b/cpus-common.c
> index 59f751ecf9..7b859752ea 100644
> --- a/cpus-common.c
> +++ b/cpus-common.c
> @@ -192,7 +192,7 @@ void start_exclusive(void)
> smp_mb();
> running_cpus = 0;
> CPU_FOREACH(other_cpu) {
> - if (atomic_read(&other_cpu->running)) {
> + if (atomic_read(&other_cpu->running) &&
> !qemu_cpu_is_self(other_cpu)) {
The comment above reads:
Must only be called from outside cpu_exec.
So we need to revise this comment. Is this really a limitation or was it
originally the design goal?
> other_cpu->has_waiter = true;
> running_cpus++;
> qemu_cpu_kick(other_cpu);
> diff --git a/cpus.c b/cpus.c
> index e1b82bcd49..981f23d52b 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -1461,10 +1461,6 @@ static void *qemu_tcg_cpu_thread_fn(void *arg)
> */
> g_assert(cpu->halted);
> break;
> - case EXCP_ATOMIC:
> - qemu_mutex_unlock_iothread();
> - cpu_exec_step_atomic(cpu);
> - qemu_mutex_lock_iothread();
> default:
> /* Ignore everything else? */
> break;
--
Alex Bennée
