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[Qemu-devel] Rationalising exit_request, cpu->exit_request and tcg_exit_
From: |
Alex Bennée |
Subject: |
[Qemu-devel] Rationalising exit_request, cpu->exit_request and tcg_exit_req? |
Date: |
Wed, 16 Dec 2015 17:14:46 +0000 |
User-agent: |
mu4e 0.9.15; emacs 24.5.50.4 |
Hi,
While looking at Fred's current MTTCG WIP branch I ran into a problem
where:
- async_safe_work_pending() was true
- this triggered setting cpu->exit_request
- however we never left tcg_exec_all()
- because the global exit_request wasn't set
- hence qemu_tcg_wait_io_event() never drained the async work queue
While trying to understand why we have both a cpu and a global
exit_request I then discovered there is also cpu->tcg_exit_req which is
the actual variable the TCG examines. This leads to sequences like:
void cpu_exit(CPUState *cpu)
{
cpu->exit_request = 1;
/* Ensure cpu_exec will see the exit request after TCG has exited. */
smp_wmb();
cpu->tcg_exit_req = 1;
}
which itself is amusingly called from:
static void qemu_cpu_kick_no_halt(void)
{
CPUState *cpu;
/* Ensure whatever caused the exit has reached the CPU threads before
* writing exit_request.
*/
atomic_mb_set(&exit_request, 1);
cpu = atomic_mb_read(&tcg_current_cpu);
if (cpu) {
cpu_exit(cpu);
}
}
This seems to me to be slightly insane as we now have 3 variables that
struggle to be kept in sync. Could all this not be rationalised into a
single variable or am I missing a subtly in their different semantics?
One problem I can think of when we get to the MTTCG world is a race when
signalling other CPUs to exit and making sure that request is not
dropped as we clear an old exit_request.
The other complication is the main cpu_exec loop which works hard to
avoid leaving the main loop when processing interrupts (which require
an exit_request to trigger). This means there a potentially multiple
places where exit_requests are drained.
I don't know if there is clean-up that can happen in master or if this
all needs to be done in the mttcg work but would it make sense just to
keep cpu->exit_request, make it visible to the TCG code and make all
exits fall out to qemu_tcg_cpu_thread_fn which would be the only place
to clear the flag?
I did have a brief look at the KVM side of the code and it only seems to
reference cpu->exit_request so I think the rest of this is a TCG
problem.
Thoughts?
--
Alex Bennée
- [Qemu-devel] Rationalising exit_request, cpu->exit_request and tcg_exit_req?,
Alex Bennée <=