Re: [Xen-devel] [PATCH 06/12 v3] xen/arm: vpl011: Add a new vuart node in the xenstore

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, May 22, 2017 at 04:33:15PM +0530, Bhupinder Thakur wrote:
> Hi Wei,
> 
> 
> > [...]
> >> >> @@ -151,13 +154,19 @@ retry_transaction:
> >> >>      if (rc) goto out;
> >> >>
> >> >>      if (!libxl_only) {
> >> >> -        rc = libxl__xs_write_checked(gc, t, 
> >> >> GCSPRINTF("%s/frontend",libxl_path),
> >> >> -                                     frontend_path);
> >> >> -        if (rc) goto out;
> >> >> +        if (fents || ro_fents)
> >> >> +        {
> >> >> +            rc = libxl__xs_write_checked(gc, t, 
> >> >> GCSPRINTF("%s/frontend",libxl_path),
> >> >> +                                         frontend_path);
> >> >> +            if (rc) goto out;
> >> >> +        }
> >> >>
> >> >> -        rc = libxl__xs_write_checked(gc, t, 
> >> >> GCSPRINTF("%s/backend",libxl_path),
> >> >> -                                     backend_path);
> >> >> -        if (rc) goto out;
> >> >> +        if (bents)
> >> >> +        {
> >> >> +            rc = libxl__xs_write_checked(gc, t, 
> >> >> GCSPRINTF("%s/backend",libxl_path),
> >> >> +                                         backend_path);
> >> >> +            if (rc) goto out;
> >> >> +        }
> >> >
> >> > What is this for?
> >> >
> >> > If there is no fe or be entries you skip the path creation altogether.
> >> > But why? This doesn't seem to be related to your patch.
> >> For vuart, I am adding only a front end node but the
> >> libxl__device_generic_add() creates the backend path also,even though
> >> there is no backend node. To remove that hanging be path, I added this
> >> check.
> >> >
> >> > At least explain this a bit in the commit message?
> >> I will add more details in the commit message.
> >>
> >
> > Preferable it should be in a separate patch. That would make review
> > easier.
> >
> > But there is another question: how do you know if Dom0 is servicing a
> > DomU? How do you construct a libxl__device struct should you want to
> > manipulate vuart?
> >
> Can you please elaborate more on this question? I am adding the vuart
> console node at the same place where the PV console node is added. I
> believe, the check that I have added should be a generic check valid
> for any device creation.
> 

Sorry if that question confuses you. Let me try to rephrase.

Nowadays you can use xl block-list / network-list etc do list respective
devices for a DomU. The underlying libxl functions normally rely on
xenstore for device information.

Now AIUI (but I could be wrong!) you don't have an backend entry for
vuart in xenstore. In a hypothetical situation, we want to add a xl
console-list to list all consoles for a DomU, where should libxl get
relevant information from?

Wei.

> Regards,
> Bhupinder

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