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Re: [Qemu-discuss] Handling signal of Qemu thread

From: Peter Maydell
Subject: Re: [Qemu-discuss] Handling signal of Qemu thread
Date: Mon, 20 Aug 2018 16:35:36 +0100

On 20 August 2018 at 16:07, Probir Roy <address@hidden> wrote:
>> What exactly are you trying to do
>> with your new signal ?
> I am implementing PEBS (Intel's Precise-Event Based Sampling)
> virtualization, so that I can sample guest OS from host machine using
> Linux Perf. The PEBS device is configured from host's user space as
> perf event. I am registering a perf-event using "perf_event_open" in
> kvm_cpu_exec:
> +++ accel/kvm/kvm-all.c    2018-08-07 11:01:05.326280431 -0500
>    @@ -1903,6 +1904,12 @@
>      qemu_mutex_unlock_iothread();
>      cpu_exec_start(cpu);
> +    /* Probir: vcpu thread starting. Should call the tool thread handler*/
> +    kvm__VCPU_start(); // calling perf_event_open
> "kvm__VCPU_start" calling perf_event_open and a signal handler
> "generic_dev_signal_handler" is set via sigaction:
> // Set a signal handler for SIGUSR1
>         struct sigaction sa;
>         sa.sa_sigaction = generic_dev_signal_handler;
>         sa.sa_mask = block_mask_dev;
>         sa.sa_flags = SA_SIGINFO | SA_RESTART | SA_NODEFER | SA_ONSTACK;

You cannot use SIGUSR1 for this. QEMU uses it internally as
its SIG_IPI (inter-vcpu-communication). You'll need to get
the perf code to use a different signal. (And you'll need to
manually unblock that signal on the CPU thread when you install
your handler.)

> I am expecting to perform some task in "generic_dev_signal_handler" at
> PEBS event when the VCPU is halted. This task can be reading/writing
> hardware registers at that event point.

Guest or host hardware registers? Either way, this is tricky
stuff to be trying to do in a signal handler.

-- PMM

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