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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 5 May 2017, Bhupinder Thakur wrote:
> 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?




> >
> >
> >>     0 = ""   (n0,r1)
> >>      port = "1"   (n0,r1)
> >>      ring-ref = "233475"   (n0,r1)
> >>      limit = "1048576"   (n0,r1)
> >>      type = "xenconsoled"   (n0,r1)
> >>    console = ""   (n0,r1)
> >>     backend = "/local/domain/0/backend/console/1/0"   (n0,r1)
> >>     backend-id = "0"   (n1,r0)
> >>     limit = "1048576"   (n0,r1)
> >>     type = "xenconsoled"   (n0,r1)
> >>     output = "pty"   (n0,r1)
> >>     tty = "/dev/pts/0"   (n0,r1)
> >>     port = "3"   (n0,r1)
> >>     ring-ref = "233472"   (n0,r1)


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