Re: [Query] Flow of PCI device dedicated to a domU

[Andrew Cooper <andrew.cooper3@citrix.com>]

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On 16/07/14 07:12, Xenbegn developer wrote:
> Hi,
> I am trying to understand the flow of how a PCI device driver in domU
> works after a PCI device is is assigned to a domU.
>
> a) If a PCI device is assigned to a domU, this device has to be on a
> PCI bus. So as per my view xen would have to somehow provide a PCI
> Controller on which this device is attached.
> => Is my assumption correct ? If yes how it is done, No then also How
> enumeration of this device happens in domU kernel

No.

PV guests have no PCI root ports/bridges; they use devices as single
entities knowing that Xen/dom0 takes care of the other bits. HVM guests
have their PCI devices attached to the virtual southbridge which is all
emulated by Qemu.

>
> b) Is the Configuration space of the PCI device directly accessible to
> the domU (assuming the kernel accesses it using memory map) ? If not
> then it is trapped by xen

All configuration space is trap+emulate in Xen, although almost all
operations permitted.

>
> c) Who assigns the MSI (addr + value) in the PCI device. If Xen then
> how Xen does a translation from Physical MSI to guest MSI (where in code)

Xen controls all interrupts on the system, which is why it needs to trap
all config accesses to notice when a domain is attempting to change the
interrupt information.  In that case, Xen fixes up its delivery of
interrupts to the guest, but leaves the underlying interrupt information
intact.

~Andrew

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