[Xen-devel] NMI SERR interrupts in dom0

[Gábor PÉK <pek@xxxxxxxxx>]

I have an Intel e1000e NIC which I put into passthrough for an HVM
domain under Xen 4.2. All the corresponding hardware protections are
enabled on my system (DMA + Interrupt remapping), however, once in a
while I get a SERR NMI in dom0 (NMI - PCI sys error (SERR) in xl dmesg).
I am wondering about its exact reason. I am thinking in the following way:

[+] Under Intel VT-x, interrupts are virtualized, thus all the
interrupts coming from my HVM domain should be handled by the
corresponding handler in dom0/(hypervisor?) which decides whether to
neglect that interrupt or inject back into the guest to be handled by
the corresponding vcpu. Thus, every time an interrupt is generated
inside my HVM guest, a VM exit occurs that is handled by the hypervisor
which forwards the "event" to the appropriate handler (in my case it is
the NMI handler in dom0 with ring1 privileges).

[+] At the same time, PCI SERR interrupts refer to hardware errors that
is generated by my passthrough NIC directly, so I expect that these
interrupts are physical (e.g., MSIs) so they should go directly either
to the BSP or one of the APs. However, Interrupt remapping is in place
which should check the origin of such interrupts and should remap the
interrupts by using the BDF id of the device. Thus, the real interrupt
is generated by the Interrupt Remapping hardware unit which is still a
physical one. Am I right?

So I have the feeling that actually it is the normal behaviour of Xen,
but it is a bit weird for me that a PT device, which can also be
controlled by a guest, could invoke an interrupt handler
out-of-the-guest with higher privileges.

Can anyone clarify this issue for me if I have a misunderstanding?

Thank you!
-gabor

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