Re: [Xen-devel] [PATCH RFC 3/4] xen/pvhvm: Unmap all PIRQs on startup and shutdown

From: David Vrabel
Date: Tue Jul 29 2014 - 11:26:05 EST

On 29/07/14 14:50, Vitaly Kuznetsov wrote:
> David Vrabel <david.vrabel@xxxxxxxxxx> writes:
>> On 15/07/14 14:40, Vitaly Kuznetsov wrote:
>>> When kexec is being run PIRQs from Qemu-emulated devices are still
>>> mapped to old event channels and new kernel has no information about
>>> that. Trying to map them twice results in the following in Xen's dmesg:
>>> (XEN) irq.c:2278: dom7: pirq 24 or emuirq 8 already mapped
>>> (XEN) irq.c:2278: dom7: pirq 24 or emuirq 12 already mapped
>>> (XEN) irq.c:2278: dom7: pirq 24 or emuirq 1 already mapped
>>> ...
>>> and the following in new kernel's dmesg:
>>> [ 92.286796] xen:events: Failed to obtain physical IRQ 4
>>> The result is that the new kernel doesn't recieve IRQs for Qemu-emulated
>>> devices. Address the issue by unmapping all mapped PIRQs on kernel shutdown
>>> when kexec was requested and on every kernel startup. We need to do this
>>> twice to deal with the following issues:
>>> - startup-time unmapping is required to make kdump work;
>>> - shutdown-time unmapping is required to support kexec-ing non-fixed kernels;
>>> - shutdown-time unmapping is required to make Qemu-emulated NICs work after
>>> kexec (event channel is being closed on shutdown but no PHYSDEVOP_unmap_pirq
>>> is being performed).
>> I think this should be done only in one place -- just prior to exec'ing
>> the new kernel (including kdump kernels).
> Thank you for your comments!
> The problem I'm fighting wiht atm is: with FIFO-based event channels we
> need to call evtchn_fifo_destroy() so next EVTCHNOP_init_control won't
> fail. I was intended to put evtchn_fifo_destroy() in
> EVTCHNOP_reset. That introduces a problem: we need to deal with
> store/console channels. It is possible to remap those from guest with
> EVTCHNOP_bind_interdomain (if we remember where they were mapped before)
> but we can't do it after we did evtchn_fifo_destroy() and we can't
> rebind them after kexec and performing EVTCHNOP_init_control as
> we can't remember where these channels were mapped to after kexec/kdump.
> I see the following possible solutions:
> 1) We put evtchn_fifo_destroy() in EVTCHNOP_init_control so
> EVTCHNOP_init_control can be called twice. No EVTCHNOP_reset is required
> in that case.

EVTCHNOP_init_control is called for each VCPU so I can't see how this
would work.

> 2) Introduce special (e.g. 'EVTCHNOP_fifo_destroy') hypercall to do
> evtchn_fifo_destroy() without closing all channels. Alternatively we can
> avoid closing all channels in EVTCHNOP_reset when called with DOMID_SELF
> (as this mode is not being used atm) -- but that would look unobvious.

I would try this. The guest prior to kexec would then:

1. Use EVTCHNOP_status to query remote end of console and xenstore event

2. Loop for all event channels:

a. unmap pirq (if required)
b. EVTCHNOP_close

3. EVTCHNOP_fifo_destroy (the implementation of which must verify that
no channels are bound).

4. EVTCHNOP_bind_interdomain to rebind the console and xenstore channels.

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