Re: [PATCH v6 2/2] arm/efi: load dom0 modules from DT using UEFI

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 12 Oct 2021, Luca Fancellu wrote:
> > On 12 Oct 2021, at 02:31, Stefano Stabellini <sstabellini@xxxxxxxxxx> wrote:
> > 
> > On Mon, 11 Oct 2021, Julien Grall wrote:
> >> Hi Stefano,
> >> 
> >> On 11/10/2021 22:24, Stefano Stabellini wrote:
> >>>> diff --git a/xen/arch/arm/efi/efi-boot.h b/xen/arch/arm/efi/efi-boot.h
> >>>> index 840728d6c0..076b827bdd 100644
> >>>> --- a/xen/arch/arm/efi/efi-boot.h
> >>>> +++ b/xen/arch/arm/efi/efi-boot.h
> >>>> @@ -713,10 +713,12 @@ static int __init 
> >>>> handle_module_node(EFI_FILE_HANDLE
> >>>> dir_handle,
> >>>>      char mod_string[24]; /* Placeholder for module@ + a 64-bit number +
> >>>> \0 */
> >>>>      int uefi_name_len, file_idx, module_compat;
> >>>>      module_name *file;
> >>>> +    const char *compat_string = is_domu_module ? "multiboot,module" :
> >>>> +                                "xen,multiboot-module";
> >>>>        /* Check if the node is a multiboot,module otherwise return */
> >>>>      module_compat = fdt_node_check_compatible(fdt, module_node_offset,
> >>>> -                                              "multiboot,module");
> >>>> +                                              compat_string);
> >>>>      if ( module_compat < 0 )
> >>>>          /* Error while checking the compatible string */
> >>>>          return ERROR_CHECK_MODULE_COMPAT;
> >>> 
> >>> 
> >>> Well... not exactly like this because this would stop a normal
> >>> "multiboot,module" dom0 kernel from being recognized.
> >>> 
> >>> So we need for domU: only "multiboot,module"
> >>> For Dom0, either "multiboot,module" or "xen,multiboot-module"
> >> 
> >> Looking at the history, xen,multiboot-module has been considered as a 
> >> legacy
> >> binding since before UEFI was introduced. In fact, without this series, I
> >> believe, there is limited reasons for the compatible to be present in the 
> >> DT
> >> as you would either use grub (which use the new compatible) or xen.cfg (the
> >> stub will create the node).
> >> 
> >> So I would argue that this compatible should not be used in combination 
> >> with
> >> UEFI and therefore we should not handle it. This would make the code 
> >> simpler
> >> :).
> > 
> 
> Hi Stefano,
> 
> > What you suggested is a viable option, however ImageBuilder is still
> > using the "xen,multiboot-module" format somehow today (no idea why) and
> > we have the following written in docs/misc/arm/device-tree/booting.txt:
> > 
> >     Xen 4.4 supported a different set of legacy compatible strings
> >     which remain supported such that systems supporting both 4.4
> >     and later can use a single DTB.
> > 
> >     - "xen,multiboot-module" equivalent to "multiboot,module"
> >     - "xen,linux-zimage"     equivalent to "multiboot,kernel"
> >     - "xen,linux-initrd"     equivalent to "multiboot,ramdisk"
> > 
> >     For compatibility with Xen 4.4 the more specific "xen,linux-*"
> >     names are non-optional and must be included.
> > 
> > My preference is to avoid breaking compatibility (even with UEFI
> > booting). The way I suggested above is one way to do it.
> > 
> > But I don't feel strongly about this at all, I am fine with ignoring
> > "xen,multiboot-module" in the EFI stub. I can get ImageBuilder fixed
> > very quickly (I should do that in any case). If we are going to ignore
> > "xen,multiboot-module" then we probably want to update the text in
> > docs/misc/arm/device-tree/booting.txt also.
> 
> The changes to support legacy compatible strings can be done but it will 
> result in
> complex code, I would go for Julien suggestion to just drop it for UEFI.
> 
> I can add a note on docs/misc/arm/device-tree/booting.txt saying that for 
> UEFI boot
> the legacy strings are not supported.
> 
> Something like:
> 
> --- a/docs/misc/arm/device-tree/booting.txt
> +++ b/docs/misc/arm/device-tree/booting.txt
> @@ -51,6 +51,8 @@ Each node contains the following properties:
>         Xen 4.4 supported a different set of legacy compatible strings
>         which remain supported such that systems supporting both 4.4
>         and later can use a single DTB.
> +       However when booting Xen using UEFI and Device Tree, the legacy 
> compatible
> +       strings are not supported.
>  
>         - "xen,multiboot-module" equivalent to "multiboot,module"
>         - "xen,linux-zimage"     equivalent to "multiboot,kernel”
> 
> 
> What do you think about that?

Also reading Julien's reply, I am fine with a doc-only change in a
separate patch.

Yes, something along those lines is OK.

So for this patch:

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>