Re: [PATCH v6 3/3] arm/libxl: Emulated PCI device tree node in libxl [and 1 more messages]

[Ian Jackson <iwj@xxxxxxxxxxxxxx>]

(Replying to both the earlier subthread on v5 and to the new v6
patch.)

Bertrand Marquis writes ("Re: [PATCH v5 10/11] arm/libxl: Emulated PCI device 
tree node in libxl"):
> Now you suggest to add a new field arm_vpci in libxl__domain_create_state.

Hi.  I was handwaving, hence "probably" :-).  I hadn't actually looked
at the existing code to see precisely how it would fit.

> Once we have done that I will need to access this structure to know if I need
> to add the DT part and somehow to give it a value depending something which
> for now would the number of pcidevs as there will be no user parameter 
> anymore.

Right.

I looked at libxl_arm.c again:

It seems that the main entrypoint to all this is libxl__prepare_dtb,
and it is that function you want to do new stuff.  That gets
libxl_domain_build_info (which is specified by the IDL and comes from
outside libxl, subject to libxl's default-filling machinery) and
libxl__domain_build_state (which is the general state struct).

The information you need is is libxl_domain_create_info.
libxl__domain_config_setdefault already arranges to set
c_info->passthrough based on the number of PCI PT devices
(search for `need_pt` in libxl_create.c).

That is, as I understand it on ARM vpci should be enabled if
passthrough is enabled and not otherwise.  That is precisely what
the variable c_info->passthrough is.

There is a slight issue because of the distinction between create_info
and build_info and domain_config (and, the corresponding _state)
structs.  Currently the DT code ony gets b_info, not the whole
libxl_domain_config.  These distinctions largely historical nowadays.
Certainly there is no reason not to pass a pointer to the whole
libxl_domain_config, rather than just libxl_domain_build_info, into
libxl__arch_domain_init_hw_description.

So I think the right approach for this looks something like this:

1. Change libxl__arch_domain_init_hw_description to take
   libxl_domain_config* rather than libxl_domain_build_info*.
   libxl_domain_config contains libxl_domain_build_info so
   this is easy.

   If you put in a convenience alias variable for the
   libxl_domain_build_info* you can avoid extra typing in the function
   body.  (If you call the convenience alias `info` you won't need to
   change the body at all, but maybe `info` isn't the best name so you
   could rename it to `b_info` maybe; up to you.)

2. Make the same change to libxl__prepare_dtb.

3. Now you can use d_config->c_info.passthrough to gate the addition
   of the additional stuff to the DT.  Ie, that is a respin of this
   patch 3/3.

1 and 2 need to be separated out from 3, in a different patch (or two
different patches) added to this series before what is now 3/3.  Ie
1+2, or 1 and 2 separately, should be pure plumbing changes with no
functional change.

I hope this is helpful.

Thanks,
Ian.