Re: [Qemu-devel] [PATCH v5 3/4] Plumb the HAXM-based hardware acceleration support

[Paolo Bonzini <pbonzini@redhat.com>]

On 05/01/2017 15:01, Paolo Bonzini wrote:
> 
> 
> On 05/01/2017 14:50, Vincent Palatin wrote:
>> Sorry I missed it.
>> I move it to qemu_cpu_kick() as asked in the Darwin patch.
>>
>>> Apart from the above change, can you check if there are some less
>>> heavyeight methods to force an exit?  I can think of QueueUserAPC with
>>> an empty pfnAPC here, and SleepEx(0, TRUE) in qemu_hax_cpu_thread_fn
>>> before qemu_wait_io_event_common.
>>
>> Actually I don't know a good test case to verify such a change, any advice ?

In fact there is a race anyway:

        if (cpu->exit_request) {
            ret = 1;
            break;
        }
					cpu->exit_request
					SuspendThread
					ResumeThread
        hax_vcpu_interrupt(env);
        qemu_mutex_unlock_iothread();
        hax_ret = hax_vcpu_run(vcpu);

and the same race is true for QueueUserAPC.  It's rare enough that I
guess we can accept the patches with just a FIXME comment, but...  Yu
Ning, can you tell us what user_event_pending is for? :)  My hunch is
that we should call hax_raise_event after setting cpu->exit_request, like

	hax_raise_event();
	/* write user_event_pending before exit_request */
	smp_wmb();
	cpu->exit_request = 1;
	SuspendThread/ResumeThread
		(or QueueUserAPC)

and in the hax thread:

        if (cpu->exit_request) {
	    cpu->hax_vcpu->tunnel->user_event_pending = 0;
            ret = 1;
            break;
        }

        hax_vcpu_interrupt(env);
        qemu_mutex_unlock_iothread();

	/* read exit_request before user_event_pending */
	smp_rmb();
        hax_ret = hax_vcpu_run(vcpu);

but I would like some more official documentation than my own reverse
engineering of the brain of whoever wrote the interface (I have not
looked at the HAXM driver binary).

Paolo