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

[Xenbegn developer <xen.begn.dev@gmail.com>]

On 7/16/14, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
> 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.
>
How would a then a PV guest know about existence of a PCI device if it
does not enumerate. Can you please elaborate on this.


>>
>> 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
>