RE: [Xen-devel] [PATCH]vtd: Fix for irq bind failure after PCI attaching 32 times

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 27 Jan 2011, Zhang, Fengzhe wrote:
> Hi, Stefano,
> 
> Here is the calling graph that cause the bug:
> 
> unregister_real_device (ioemu)
>     |
>     +----> pt_msix_disable (ioemu)
>             |
>             +----> xc_domain_unbind_msi_irq (ioemu)
>             |       |
>             |       +----> do_domctl (xen) ----> arch_do_domctl (xen) ----> 
> pt_irq_destroy_bind_vtd (xen)
>             |              |
>             |              +----> unmap_domain_pirq_emuirq (xen)  //freed 
> pirq_to_emuirq
>             |
>             +----> xc_physdev_unmap_pirq (ioemu)
>                    |
>                    +----> do_physdev_op (xen) 
>                            |
>                            +----> physdev_unmap_pirq (xen)
>                                    |
>                                    +----> unmap_domain_pirq_emuirq (xen)  
> //found pirq_to_emuirq already freed, abort
>                                    |
>                                    +----> unmap_domain_pirq (xen)    //not 
> called
> 
> The code path you mentioned is not taken for VF dev as its ptdev->machine_irq 
> is 0.

It has just occurred to me that all this only happens with guest
cooperation: unregister_real_device is called on pci hotplug in response
to guest's action.
That means that a guest that doesn't support pci hot-unplug (or a
malicious guest) won't do anything in response to the acpi SCI interrupt
we send, therefore unregister_real_device will never be called and we
will be leaking MSIs in the host!

Of course we could solve it adding a new xenstore command to qemu that
calls unregister_real_device directly, but it seems to me that relying
on qemu to free some hypervisor/dom0 resources is not a good idea.

Xen knows all the pirq remapped to this domain, so wouldn't it be
possible for Xen to call pt_irq_destroy_bind_vtd and physdev_unmap_pirq
on domain_kill?
I think that Xen shouldn't leak pirqs no matter what the toolstack or
qemu do.

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