Question on pci-passthrough and event delivery

Question on pci-passthrough and event delivery

[Thomas Pfeuffer]

Hello,

I am using pci-passthrough mechanism in Xen 3.4.2-rc2 to passthrough the 
only Network Interface Card to a pv driver domain.

I have observed in xentrace output, that everytime a network packet 
arrives, the Xen hypervisor delivers an event to the driver domain and 
to dom0. I would think, that it is not neccassary to inform dom0, since 
only the driver domain has access to the NIC.

Why is the interrupt from the NIC delivered to dom0 via an event, too? 
Is it neccassary for pciback in dom0?

regards,

Thomas

Re: Question on pci-passthrough and event delivery

[Konrad Rzeszutek Wilk]

On Thu, Apr 08, 2010 at 10:48:03AM +0200, Thomas Pfeuffer wrote:
> Hello,
>
> I am using pci-passthrough mechanism in Xen 3.4.2-rc2 to passthrough the  
> only Network Interface Card to a pv driver domain.
>
> I have observed in xentrace output, that everytime a network packet  
> arrives, the Xen hypervisor delivers an event to the driver domain and  
> to dom0. I would think, that it is not neccassary to inform dom0, since  
> only the driver domain has access to the NIC.
>
> Why is the interrupt from the NIC delivered to dom0 via an event, too?  

The primary reason is that the SHARED_IRQ flag is set for the IRQ so
Xen delievers the IRQ to both Dom0 and the DomU which has the PCI device.

This is neccessary when there are more than one device sharing the same
IRQ (and they are in different domains). For situations in which there
is only device using the IRQ, this isn't neccessary - in fact, I think
this might be the bug that has been plaguing some users.

> Is it neccassary for pciback in dom0?

No. The events between PCI back and PCI front are via their own little
mechanism and it is only be used when a PCI device writes configuration
data or changes some state. Normal opperations, should not trigger this.