Re: [Xen-devel] [XEN PATCH v1] libxl: Add DTB compatible list to config file

[Oleksandr Grytsov <al1img@xxxxxxxxx>]

On Wed, Oct 16, 2019 at 5:12 PM Julien Grall <julien.grall@xxxxxxx> wrote:
>
> Hi Oleksandr,
>
> On 16/10/2019 15:04, Oleksandr Grytsov wrote:
> > On Fri, Oct 11, 2019 at 8:21 PM Stefano Stabellini
> > <sstabellini@xxxxxxxxxx> wrote:
> >>
> >> On Fri, 11 Oct 2019, Julien Grall wrote:
> >>> Hi,
> >>>
> >>> On 11/10/2019 16:23, Ian Jackson wrote:
> >>>> Oleksandr Grytsov writes ("[PATCH v1] libxl: Add DTB compatible list to
> >>>> config file"):
> >>>>> From: Oleksandr Grytsov <oleksandr_grytsov@xxxxxxxx>
> >>>>>
> >>>>> Some platforms need more compatible property values in device
> >>>>> tree root node in addition to "xen,xenvm-%d.%d" and "xen,xenvm"
> >>>>> values that are given by Xen by default.
> >>>
> >>> I am pretty sure I have seen this patch a few years ago, but I can't find 
> >>> it
> >>> in my inbox. What is the exact problem here?
> >>>
> >>>>> Specify in domain configuration file which values should be added
> >>>>> by providing "dtb_compatible" list of strings separated by comas.
> >>>>
> >>>> Hi, thanks.
> >>>>
> >>>> I don't have an opinion about the principle of this and would like to
> >>>> hear from ARM folks about that.
> >>>>
> >>>> Also, Stefano, Julien: should we be asking for a freeze exception for
> >>>> this for 4.13 ?
> >>>
> >>> I don't have enough context to understand the exact issue he is trying to
> >>> solve.
> >>
> >> Same here. Is this patch needed because on some platform the OS checks
> >> for the platform "model" (also known as "machine name") on device tree
> >> before continuing or to trigger a difference behavior?
> >
> > Yes, exactly.
> >
> > I will redo the patch with Ian's comments if it is ok in general.
>
> By specifying a different compatible (let say "renesas,r8a774a1"), then you
> claim that your virtual machine is exactly the same as that board.
>
> This means, the OS is free to assume that the memory layout and all the 
> devices
> are exactly the same. This is definitely not true as the virtual machine we
> expose is specific to Xen.
>
> So I don't think this is the correct approach here. Can you provide a 
> real-life
> example on why you need this?
>
> Cheers,
>
> --
> Julien Grall

This is required for HW domains. Some drivers or initialization routines check
compatible. Below is example from linux kernel sources:

linux/sound/ppc/awacs.c:741:    if (of_machine_is_compatible("PowerBook3,1")
linux/sound/ppc/awacs.c:742:        ||
of_machine_is_compatible("PowerBook3,2")) {
linux/sound/ppc/awacs.c:770:#define IS_PM7500
(of_machine_is_compatible("AAPL,7500") \
linux/sound/ppc/awacs.c:771:        || of_machine_is_compatible("AAPL,8500") \
linux/sound/ppc/awacs.c:772:        || of_machine_is_compatible("AAPL,9500"))
...
linux/arch/arm/mach-omap2/pdata-quirks.c:703:        if
(of_machine_is_compatible(quirks->compatible)) {
linux/arch/arm/mach-omap2/pdata-quirks.c:717:    if
(of_machine_is_compatible("ti,omap2420") ||
linux/arch/arm/mach-omap2/pdata-quirks.c:718:
of_machine_is_compatible("ti,omap3"))
linux/arch/arm/mach-omap2/pdata-quirks.c:721:    if
(of_machine_is_compatible("ti,omap3"))
...

Also see [1]

[1] 
https://source.codeaurora.org/external/imx/imx-xen/commit/?h=imx_4.14.98_2.0.0_ga&id=6e58d478203639e71da3da69ffeae3fa5dc0197b

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