Re: [Xen-devel] Re: APIC rework

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

> I am also confusing about how to assign a device to a PV guest. Originally  I 
> thought the logic should be it should be hidden first through pciback in 
> dom0, and pv guest uses its pci subsystem (pcifront end) to require pciback 
> to mapping this device.  In the first step, once pciback.hide logic is called 
> in dom0,  this device should be released by dom0 and avaliable for PV guests 
> at this time. And the in subsequent step, pciback or dom0's pci system should 
> help PV guests to do the irq mapping, otherwise I can't see how the irqs from 
> the assigned device are delivered to PV guests.   

That is correct. Here is an URL of the correct steps:
http://wiki.xensource.com/xenwiki/Assign_hardware_to_DomU_with_PCIBack_as_module

>From the unprivileged side (domU), when it makes a call to
pci_enable_device, it gets routed to pcifront_bus_write.
On the privileged side (dom0), pciback picks up the write
and routes it to 'command_write' (conf_space_header.c). It
does the pci_enable_device in the dom0 side. The pci_enable_device
ends up calling xen_allocate_pirq which gets the IRQ from
PHYSDEVOP_alloc_irq_vector. That IRQ is saved in dev->irq and
is visible to the DomU.

Also during the pci_enable_device (in the DomU side), pcibios_enable_device
gets called - which in domU is called 'xen_pcifront_enable_irq'.
The xen_pcifront_enable_irq allocates an irq_desc with xen_pirq_chip
structure. The GSI it requests is actually the IRQ number from dev->irq.

To summarize, dom0 on behalf of domU, calls PHYSDEVOP_alloc_irq_vector
for the device in question. Saves the GSI in dev->irq which is visible to
the DomU. DomU sets up a xen_pirq_chip structure for the device and
starts/stop/etc through that function structure. Please note that 
there is nothing in PHYSDEVOP_alloc..  about which domain owns the device.
That is only done with PHYSDEVOP_map_pirq calls.

With your patch instead of PHYSDEVOP_alloc_irq_vector, it would be
PHYSDEVOP_map_pirq.

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