Re: [Xen-devel] Question about the XEN platform pci

["karim.allah.ahmed@xxxxxxxxx" <karim.allah.ahmed@xxxxxxxxx>]

On Tue, Apr 12, 2016 at 5:45 PM, Konrad Rzeszutek Wilk
<konrad.wilk@xxxxxxxxxx> wrote:
> On Tue, Apr 12, 2016 at 05:33:47PM +0200, karim.allah.ahmed@xxxxxxxxx wrote:
>> The INTx interrupt of this platform device can be used by Xen in HVM case to
>> notify the guest of pending events in the event channel. However that's 
>> usually
>> not used in favor of vector callbacks support in Xen where a vector is 
>> injected
>> directly to the vCPU bypassing LAPIC.
>>
>> (that said, the platform-pci driver in linux is actually broken when vector
>> callbacks are not used anyway)
>
> Oh? Is there an report/bug somewhere?
>

I'm not sure if it's reported as a bug somewhere or not.

I've always assumed that INTx is deperecated and vector callbacks are the one
that's supported that's why I never tried to fix it, in addition I never tried
to reproduce it I just looked at the code and it seemed a little bit off as
explained below.

Mainly xenbus_init is called during postcore_initcall which will eventually try
to read a value from XenStore and will get stuck on read_reply at xenbus
forever since the platform driver is not probed yet and its INTx interrupt
handler is not registered yet which basically means that the guest can not be
notified at this moment of any pending event channels and none of the per-event
handlers will ever be invoked (including the XenStore one) and the reply will
never be picked up by the kernel.

The exact stack where things get stuck during xenbus_init:

-xenbus_init
 -xs_init
  -xs_reset_watches
   -xenbus_scanf
    -xenbus_read
     -xs_single
      -xs_single
       -xs_talkv

> Thanks!
>>
>> I also think that the grant-table lives on this PCI device MMIO BAR (?!)
>
> The area may be usurped for grant-table as the OS won't touch that memory
> area (it after all belongs to the device).
>>
>> If you looked at hw/i386/xen/xen_platform.c in QEMU source , you will get a
>> general idea what this device is supposed todo (like logging to syslog stuff
>> for example).
>>
>> That said the platform device is really not fully utilized anyway in Linux.
>>
>> On Tue, Apr 12, 2016 at 4:09 PM, Konrad Rzeszutek Wilk
>> <konrad.wilk@xxxxxxxxxx> wrote:
>> > On Tue, Apr 12, 2016 at 02:19:48AM +0000, Wu, Bob wrote:
>> >>
>> >> Really thanks for your reply.
>> >
>> > Hey!
>> >
>> > CC-ing Xen-devel back on. Please do not drop it and please don't
>> > top-post.
>> >>
>> >> Can you explain a little more?
>> >
>> > I am not sure what you want me to explain. Perhaps if you
>> > read http://xenbits.xen.org/docs/unstable/misc/hvm-emulated-unplug.html
>> > it may become clearer?
>> >
>> >> Is the xen platform pci driver the only purpose for telling QEMU  that 
>> >> don’t emulate the IDE driver?
>> >
>> > And network.
>> >> I think it can be done by a simple way, but don't need use this huge 
>> >> platform driver.
>> >
>> > ?
>> >>
>> >> I guess this is for PCI pass through in XEN HVM mode, but don't sure.
>> >
>> > No.
>> >>
>> >> Thanks,
>> >> Bob
>> >>
>> >>
>> >> -----Original Message-----
>> >> From: Konrad Rzeszutek Wilk [mailto:konrad.wilk@xxxxxxxxxx]
>> >> Sent: 2016年4月11日 22:24
>> >> To: Wu, Bob
>> >> Cc: xen-devel@xxxxxxxxxxxxx
>> >> Subject: Re: [Xen-devel] Question about the XEN platform pci
>> >>
>> >> On Fri, Apr 08, 2016 at 08:52:08AM +0000, Wu, Bob wrote:
>> >> >
>> >> > Sorry bother, I read the XEN source code recently, and found the XEN
>> >> > platform PCI code under drivers/xen/platform-pci.c, and I can't fully 
>> >> > under this driver's affect, can anybody explain a little for me?
>> >> >
>> >> > Is the platform PCI driver for PV-split-PCI-driver-model such as the 
>> >> > pci-frontend/pci-backend? or for PCI pass-through model? Or for other 
>> >> > purpose?
>> >> > I saw the xenbus_pcifront_driver/ xenbus_xen_pcibk_driver are 
>> >> > registered on XENBUS, so I guess the platform-PCI-driver is not for PV 
>> >> > PCI driver.
>> >> >
>> >>
>> >> It is for the QEMU driver. To tell QEMU to stop emulating the IDE/network.
>> >>
>> >> > Really thank you for your replay.
>> >> >
>> >> > Thanks,
>> >> > Bob
>> >> >
>> >>
>> >> > _______________________________________________
>> >> > Xen-devel mailing list
>> >> > Xen-devel@xxxxxxxxxxxxx
>> >> > http://lists.xen.org/xen-devel
>> >>
>> >
>> > _______________________________________________
>> > Xen-devel mailing list
>> > Xen-devel@xxxxxxxxxxxxx
>> > http://lists.xen.org/xen-devel
>>
>>
>>
>> --
>> Karim Allah Ahmed.



-- 
Karim Allah Ahmed.

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