Re: [PATCH V3 15/23] xen/arm: Stick around in leave_hypervisor_to_guest until I/O has completed

[Oleksandr <olekstysh@xxxxxxxxx>]

On 10.12.20 04:30, Stefano Stabellini wrote:

Hi Julien, Stefano

> On Wed, 9 Dec 2020, Julien Grall wrote:
> > On 09/12/2020 23:35, Stefano Stabellini wrote:
> > > On Wed, 9 Dec 2020, Stefano Stabellini wrote:
> > > > On Mon, 30 Nov 2020, Oleksandr Tyshchenko wrote:
> > > > > From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> > > > >
> > > > > This patch adds proper handling of return value of
> > > > > vcpu_ioreq_handle_completion() which involves using a loop
> > > > > in leave_hypervisor_to_guest().
> > > > >
> > > > > The reason to use an unbounded loop here is the fact that vCPU
> > > > > shouldn't continue until an I/O has completed. In Xen case, if an I/O
> > > > > never completes then it most likely means that something went horribly
> > > > > wrong with the Device Emulator. And it is most likely not safe to
> > > > > continue. So letting the vCPU to spin forever if I/O never completes
> > > > > is a safer action than letting it continue and leaving the guest in
> > > > > unclear state and is the best what we can do for now.
> > > > >
> > > > > This wouldn't be an issue for Xen as do_softirq() would be called at
> > > > > every loop. In case of failure, the guest will crash and the vCPU
> > > > > will be unscheduled.
> > > > Imagine that we have two guests: one that requires an ioreq server and
> > > > one that doesn't. If I am not mistaken this loop could potentially spin
> > > > forever on a pcpu, thus preventing any other guest being scheduled, even
> > > > if the other guest doesn't need any ioreq servers.
> > > >
> > > >
> > > > My other concern is that we are busy-looping. Could we call something
> > > > like wfi() or do_idle() instead? The ioreq server event notification of
> > > > completion should wake us up?
> > > >
> > > > Following this line of thinking, I am wondering if instead of the
> > > > busy-loop we should call vcpu_block_unless_event_pending(current) in
> > > > try_handle_mmio if IO_RETRY. Then when the emulation is done, QEMU (or
> > > > equivalent) calls xenevtchn_notify which ends up waking up the domU
> > > > vcpu. Would that work?
> > > I read now Julien's reply: we are already doing something similar to
> > > what I suggested with the following call chain:
> > >
> > > check_for_vcpu_work -> vcpu_ioreq_handle_completion -> wait_for_io ->
> > > wait_on_xen_event_channel
> > >
> > > So the busy-loop here is only a safety-belt in cause of a spurious
> > > wake-up, in which case we are going to call again check_for_vcpu_work,
> > > potentially causing a guest reschedule.
> > >
> > > Then, this is fine and addresses both my concerns. Maybe let's add a note
> > > in the commit message about it.
> > Damm, I hit the "sent" button just a second before seen your reply. :/ Oh
> > well. I suggested the same because I have seen the same question multiple
> > time.

I will update commit description and probably the comment in code.


--
Regards,

Oleksandr Tyshchenko