Re: [Qemu-devel] MTTCG External Halt

[Paolo Bonzini <pbonzini@redhat.com>]

On 03/01/2018 17:10, Alistair Francis wrote:
> Hey guys, I'm super stuck with an ugly MTTCG issue and was wondering
> if anyone had any ideas.
> 
> In the Xilinx fork of QEMU (based on 2.11) we have a way for CPUs to
> halt other CPUs. This is used for example when the power control unit
> halts the ARM A53s. To do this we have internal GPIO signals that end
> up calling a function that basically does this:
> 
> To halt:
>     cpu->halted = true;
>     cpu_interrupt(cpu, CPU_INTERRUPT_HALT);

cpu->halted = true should not be needed here.  It will be set by
cpu_handle_interrupt when processing CPU_INTERRUPT_HALT.

> To un-halt
>     cpu->halted = false;
>     cpu_reset_interrupt(cpu, CPU_INTERRUPT_HALT);

cpu->halted = false likewise should not be needed here, but you cannot
just clear CPU_INTERRUPT_HALT either.  You need to set a *different*
interrupt request bit (the dummy CPU_INTERRUPT_EXITTB will do) and
cpu_handle_halt will clear cpu->halted.

Paolo

> We also have the standard ARM WFI (Wait For Interrupt) implementation
> in op_helper.c:
>     cs->halted = 1;
>     cs->exception_index = EXCP_HLT;
>     cpu_loop_exit(cs);
> 
> Before MTTCG this used to work great, but now either we end up with
> the guest Linux complaining about CPU stalls or we hit:
> ERROR:/scratch/alistai/master-qemu/cpus.c:1516:qemu_tcg_cpu_thread_fn:
> assertion failed: (cpu->halted)
> 
> If I remove the instances of manually setting cpu->halted then I don't
> see the asserts(), but the the WFI instruction doesn't work correctly.
> So it seems like setting the halted status externally from the CPU
> causes the issue. I have tried setting it inside a lock, using atomic
> operations and running the setter async on the CPU, but nothing works.
> 
> Any chance any one has some insight into a way to externally set a
> vCPU as halted/un-halted?
> 
> Thanks,
> Alistair
>