Re: [PATCH] hvc/xen: fix event channel handling for secondary consoles

[Juergen Gross]

On 18.10.23 01:46, David Woodhouse wrote:
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
>
> The xencons_connect_backend() function allocates a local interdomain
> event channel with xenbus_alloc_evtchn(), then calls
> bind_interdomain_evtchn_to_irq_lateeoi() to bind to that port# on the
> *remote* domain.
>
> That doesn't work very well:
>
> (qemu) device_add xen-console,id=con1,chardev=pty0
> [   44.323872] xenconsole console-1: 2 xenbus_dev_probe on device/console/1
> [   44.323995] xenconsole: probe of console-1 failed with error -2
>
> Fix it to use bind_evtchn_to_irq_lateeoi(), which does the right thing
> by just binding that *local* event channel to an irq. The backend will
> do the interdomain binding.
>
> This didn't affect the primary console because the setup for that is
> special — the toolstack allocates the guest event channel and the guest
> discovers it with HVMOP_get_param.
>
> Once that's fixed, there's also a warning on hot-unplug because
> xencons_disconnect_backend() unconditionally calls free_irq() via
> unbind_from_irqhandler():
>
> (qemu) device_del con1
> [   32.050919] ------------[ cut here ]------------
> [   32.050942] Trying to free already-free IRQ 33
> [   32.050990] WARNING: CPU: 0 PID: 51 at kernel/irq/manage.c:1895 __free_irq+0x1d4/0x330
>
> Fix that by calling notifier_del_irq() first, which only calls
> free_irq() if the irq was requested in the first place. Then use

I don't think the "if the irq was requested in the first place" is the correct
reasoning.

I think the problem is that notifier_del_irq() will be called another time
through the .notifier_del hook. Two calls of notifier_del_irq() are fine, but
one call of it and another call of free_irq() via unbind_from_irqhandler() is
a problem.

> evtchn_put() to release the irq and event channel. Avoid calling
> xenbus_free_evtchn() in the normal case, as evtchn_put() will do that
> too. The only time xenbus_free_evtchn() needs to be called is for the
> cleanup when bind_evtchn_to_irq_lateeoi() fails.
>
> Finally, fix the error path in xen_hvc_init() when there's no primary
> console. It should still register the frontend driver, as there may be
> secondary consoles. (Qemu can always add secondary consoles, but only
> the toolstack can add the primary because it's special.)
>
> Fixes: fe415186b4 ("xen/console: harden hvc_xen against event channel storms")
> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx

With above fixed in the commit message:

Reviewed-by: Juergen Gross <jgross@xxxxxxxx>


Juergen

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