Re: [PATCH v8 07/20] multi-process: add co-routines to communicate with remote

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Jul 31, 2020 at 02:20:14PM -0400, Jagannathan Raman wrote:
> +/*
> + * Create if needed and enter co-routine to send the message to the
> + * remote channel ioc and wait for the reply.
> + * Returns the value from the reply message, sets the error on failure.
> + */
> +
> +uint64_t mpqemu_msg_send_and_await_reply(MPQemuMsg *msg, QIOChannel *ioc,
> +                                  Error **errp)
> +{
> +    MPQemuRequest req = {0};
> +    uint64_t ret = UINT64_MAX;
> +
> +    req.ioc = ioc;
> +    if (!req.ioc) {
> +        error_setg(errp, "Channel is set to NULL");
> +        return ret;
> +    }
> +
> +    req.msg = msg;
> +    req.ret = 0;
> +    req.finished = false;
> +
> +    req.co = qemu_coroutine_create(mpqemu_msg_send_co, &req);
> +    qemu_coroutine_enter(req.co);
> +
> +    while (!req.finished) {
> +        aio_poll(qemu_get_aio_context(), true);
> +    }

This is called from vcpu threads. aio_poll() does not release the global
mutex so all other vcpu threads are blocked until mpqemu communication
completes.

The simplest solution is to unlock the global mutex and call blocking,
non-coroutine versions of the send/recv code. That way other vcpu
threads can execute.

This means that the QEMU process would not use the mpqemu IOChannel in
the event loop.

The remote process would still use the IOChannel in the event loop,
however.

Stefan

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