Re: [Xen-devel] [PATCH 06/10 v2] xen/arm: vpl011: Add vuart ring-buf and evtchn to xenstore

[Bhupinder Thakur <bhupinder.thakur@xxxxxxxxxx>]

Hi Stefano,

>> >> >> >> > It looks like you are reusing the libxl__device_console_add call 
>> >> >> >> > for the
>> >> >> >> > main PV console for the domain, to also add the vuart nodes to 
>> >> >> >> > xenstore.
>> >> >> >> >
>> >> >> >> > I don't think it is a good idea to mix the two. I suggest to 
>> >> >> >> > introduce a
>> >> >> >> > new libxl__device call to introduce the vuart nodes to xenstore, 
>> >> >> >> > given
>> >> >> >> > that they have no relantionship with the principal PV console of 
>> >> >> >> > the
>> >> >> >> > domain.
>> >> >> >> >
>> >> >> >> I have a doubt here. Do I have to create a new console device
>> >> >> >> (libxl__device) to register the vuart console or can I use the
>> >> >> >> existing console device (which is used for registering the primary
>> >> >> >> console) to register the vuart nodes to xenstore?
>> >> >> >>
>> >> >> >> I suspect that if I try to register with the same console device 
>> >> >> >> then
>> >> >> >> libxl__device_generic_add() may fail as it is already added.
>> >> >> >
>> >> >> > The vuart we are introducing with this patch series is a new and
>> >> >> > different console from the existing PV console. So yes, I think we 
>> >> >> > need
>> >> >> > to create a new device for it.
>> >> >> >
>> >> >> > We either need to introduce a brand new function to add the vuart to
>> >> >> > xenstore, something like libxl__device_vuart_add, or, if we are going
>> >> >> > to reuse libxl__device_console_add, then we we need to create both a 
>> >> >> > new
>> >> >> > console device (libxl__device_console), and a new libxl__device for 
>> >> >> > it.
>> >> >> > For clarity, libxl__device_console is the input parameter of
>> >> >> > libxl__device_console_add, while libxl__device is the output 
>> >> >> > parameter.
>> >> >> >
>> >> >> I am trying to add a new vuart device by defining a new device type
>> >> >> LIBXL__DEVICE_KIND_VUART  and a new function
>> >> >> libxl__device_vuart_add().
>> >> >
>> >> > I think that is the right way to do it.
>> >> >
>> >> >
>> >> >> This function basically adds the vuart ring-ref ("0/ring-ref") and
>> >> >> port ("0/port") to ro_front array and then it registers the device
>> >> >> using the libxl__device_generic_add(). I pass NULL to "back" and
>> >> >> "front" array arguments to  libxl__device_generic_add() as I am adding
>> >> >> parameters only to "ro_front" array.
>> >> >>
>> >> >> I am adding the device at /local/domain/<domid>/vuart.
>> >> >>
>> >> >> However, xenconsoled fails to read the vuart ring-ref and port saying
>> >> >> "no such directory or file".I verified that
>> >> >> libxl__device_generic_add() is returning successfully.
>> >> >>
>> >> >> In xenconsoled, I verified that I am reading
>> >> >> /local/domain/<domid>/vuart/0/ring-ref and
>> >> >> /local/domain/<domid>/vuart/0/port.
>> >> >>
>> >> >> It is not clear why xenconsoled fails to read the values.
>> >> >
>> >> > Double check the permissions of those nodes with xenstore-ls -p. Make
>> >> > sure there are no races between libxl writing the nodes and xenconsoled
>> >> > reading them.
>> >>
>> >> xenstore-ls -p" shows that the vuart node is created successfully. I
>> >> have pasted below the relevant part of the command output:
>> >>
>> >>    vuart = ""   (n0,r1)
>> >>     backend = "/local/domain/0/backend/vuart/1/0"   (n0,r1)
>> >
>> > Why is this here instead of under vuart/0?
>> > Also, if these are the frontend nodes, where are the backend vuart nodes?
>>
>> I created only frontend nodes so that xenconsoled could access
>> ring-ref/port. I understand that backend nodes are required for guest
>> domains to access some backend information. Since in this case guest
>> domain does not need such information I did not create the backend
>> node. Kindly let me know if my understanding is correct.
>
> If there are no backend xenstore nodes, why do we have:
>
>   backend = "/local/domain/0/backend/vuart/1/0"
>
> on Xenstore?
>
libxl__device_generic_add() creates the references to backend and
frontend even if those nodes are not created (i.e. the node
information passed to the function are null). I will add a check in
libxl__device_generic_add() to add these references only if the
corresponding nodes have been created in xenstore.

Regards,
Bhupinder

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