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

From: Xenbegn developer <xen.begn.dev@gmail.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: xen-devel@lists.xen.org
Subject: Re: [Query] Flow of PCI device dedicated to a domU
Date: Tue, 22 Jul 2014 15:32:58 +0530	[thread overview]
Message-ID: <CABJY2XDm07zx8Kojsqafrrt2Jb-u_otw5HeUJ1XnqDtz_k4gZw@mail.gmail.com> (raw)
In-Reply-To: <53CE34AE.9030500@citrix.com>

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On Tue, Jul 22, 2014 at 3:23 PM, Andrew Cooper <andrew.cooper3@citrix.com>
wrote:

>  On 22/07/14 10:43, Xenbegn developer wrote:
>
>
>
>
> On Thu, Jul 17, 2014 at 3:12 PM, Andrew Cooper <andrew.cooper3@citrix.com>
> wrote:
>
>> On 17/07/14 07:34, Xenbegn developer wrote:
>> > 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.
>>
>>  The pcifront driver is capable of finding any PCI devices offered by
>> pciback in dom0.  It is all paravirtualised, and doesn't need explicit
>> hardware probing.
>>
>>  So does this flow happens internally in xen for x86
> a) dom0 scans all the pci devices
>  b) Using libxl hypercalls a pci device is assigned to a domainU
>  c) When  domU boots the pci front driver would talk to pci back driver
> and discover the PCI devices
>
>
> From a basic point of view, yes.
>
> Two points here
a) Does Xen do a PCI enumeration ? if yes then what is the use case if dom0
is capable of enumeration.
b) What other cases which are beyond the "basic point of view" where there
is a different flow

> ~Andrew
>

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