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Re: [Qemu-devel] MTTCG External Halt

From: Alistair Francis
Subject: Re: [Qemu-devel] MTTCG External Halt
Date: Wed, 31 Jan 2018 14:31:08 -0800

On Wed, Jan 31, 2018 at 12:32 PM, Alex Bennée <address@hidden> wrote:
> Alistair Francis <address@hidden> writes:
>> On Tue, Jan 30, 2018 at 8:26 PM, Paolo Bonzini <address@hidden> wrote:
>>> On 30/01/2018 18:56, Alistair Francis wrote:
>>>> I don't have a good solution though, as setting CPU_INTERRUPT_RESET
>>>> doesn't help (that isn't handled while we are halted) and
>>>> async_run_on_cpu()/run_on_cpu() doesn't reliably reset the CPU when we
>>>> want.
>>>> I've ever tried pausing all CPUs before reseting the CPU and them
>>>> resuming them all but that doesn't seem to to work either.
>>> async_safe_run_on_cpu would be like async_run_on_cpu, except that it
>>> takes care of stopping all other CPUs while the function runs.
>>>> Is there
>>>> anything I'm missing? Is there no reliable way to reset a CPU?
>>> What do you mean by reliable?  Executing no instruction after the one
>>> you were at?
>> The reset is called by a GPIO line, so I need the reset to be called
>> basically as quickly as the GPIO line changes. The async_ and
>> async_safe_ functions seem to not run quickly enough, even if I run a
>> process_work_queue() function afterwards.
>> Is there a way to kick the CPU to act on the async_*?
> Define quickly enough? The async_(safe) functions kick the vCPUs so they
> will all exit the run loop as they enter the next TB (even if they loop
> to themselves).

We have a special power controller CPU that wakes all the CPUs up and
at boot the async_* functions don't wake the CPUs up. If I just use
the cpu_rest() function directly everything starts fine (but then I
hit issues later).

If I forcefully run process_queued_cpu_work() then I can get the CPUs
up, but I don't think that is the right solution.

> From an external vCPUs point of view those extra instructions have
> already executed. If the resetting vCPU needs them to have reset by the
> time it executes it's next instruction it should either cpu_loop_exit at
> that point or ensure it is the last instruction in it's TB (which is
> what we do for the MMU flush cases in ARM, they all end the TB at that
> point).

cpu_loop_exit() sounds like it would help, but as I'm not in the CPU
context it just seg faults.


>> Thanks,
>> Alistair
>>> Paolo
> --
> Alex Bennée

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