Re: [XEN][PATCH] common/libfdt: optimize usage

[Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>]

On Fri Nov 14, 2025 at 7:01 PM CET, Grygorii Strashko wrote:
> From: Grygorii Strashko <grygorii_strashko@xxxxxxxx>
>
> Now all libfdt features are built-it unconditionally, but...
>
> X86: The libfdt is used on x86 only to parse Hyperlaunch/dom0less Xen
> nodes, so full libfdt is not needed in this case and minimal, RO
> configuration can be used.
>
> ARM - situation is more complicated:
> 1) ARM reads Host DT (fdt.c RO)
> 2) ARM reads passthrough DT (RO)
> 3) ARM generates dom0/hwdom DT from Host DT (there is a mix of WIP and SW 
> APIs)
> 4) ARM generates domU DT (there is a mix of WIP and SW APIs)
> 4) With EFI enabled - ARM needs RW API and fdt_empty_tree
> 5) With CONFIG_OVERLAY_DTB - ARM needs RW and fdt_overlay API
>
> Hence, add possibility for optimizing libfdt usage by introducing separate
> Kconfig options for each libfdt feature and select them where needed.
>
> Following libfdt modules are not used after this change:
>  Makefile.libfdt
>  fdt_addresses.c
>  fdt_strerror.c
>  fdt_check.c

This is a nice idea, and the less we compile of this nonsense library the
better.

>
> Signed-off-by: Grygorii Strashko <grygorii_strashko@xxxxxxxx>
> ---
> Not sure about using DOMAIN_BUILD_HELPERS for selecting 
> LIBFDT features, as DOMAIN_BUILD_HELPERS doesn't exactly
> says that domain's DT will be generated when selected.

We likely want a DEVICE_TREE_WRITER Kconfig, or some such. That would compile
in all code that performs DTB writes and gate the DTB overlay. Not having it
restricts libfdt to fdt.c and fdt_ro.c, and having it adds fdt_rw, and allows
the others depending on EFI and/or overlay settings.

But the helpers themselves are not terribly well organized, so I'd say it's
not strictly required for now.

>
>  xen/arch/arm/Kconfig       |  4 ++++
>  xen/common/Kconfig         |  4 ++++
>  xen/common/libfdt/Kconfig  | 14 ++++++++++++++
>  xen/common/libfdt/Makefile | 12 +++++++++---
>  4 files changed, 31 insertions(+), 3 deletions(-)
>  create mode 100644 xen/common/libfdt/Kconfig
>
> diff --git a/xen/arch/arm/Kconfig b/xen/arch/arm/Kconfig
> index cf6af68299f6..f10cd3d7effc 100644
> --- a/xen/arch/arm/Kconfig
> +++ b/xen/arch/arm/Kconfig
> @@ -111,6 +111,8 @@ config ARM_EFI
>       bool "UEFI boot service support"
>       depends on ARM_64 && !MPU
>       default y
> +     select LIBFDT_RW
> +     select LIBFDT_EMPTY_TREE

Only on UEFI? That's surprising. Why does this entry point need more than
creating a DTB for a domU in dom0less?

>       help
>         This option provides support for boot services through
>         UEFI firmware. A UEFI stub is provided to allow Xen to
> @@ -149,6 +151,8 @@ config HAS_ITS
>  config OVERLAY_DTB
>       bool "DTB overlay support (UNSUPPORTED)" if UNSUPPORTED
>       depends on SYSCTL
> +     select LIBFDT_RW
> +     select LIBFDT_OVERLAY
>       help
>         Dynamic addition/removal of Xen device tree nodes using a dtbo.
>  
> diff --git a/xen/common/Kconfig b/xen/common/Kconfig
> index 401d5046f6f5..256aff269c3b 100644
> --- a/xen/common/Kconfig
> +++ b/xen/common/Kconfig
> @@ -28,6 +28,8 @@ config DOM0LESS_BOOT
>  
>  config DOMAIN_BUILD_HELPERS
>       bool
> +     select LIBFDT_WIP
> +     select LIBFDT_SW

Out of curiosity, what's the relationship between those libfdt files and the
helpers? What do they use them for?

It's not like LIBFDT_WIP or LIBFDT_SW have very insighful names (through no
fault of your own. The original file organization is dubious at best), so why
not just compile those files conditionally on DOMAIN_BUILD_HELPERS instead?

Something like...

>  
>  config GRANT_TABLE
>       bool "Grant table support" if EXPERT
> @@ -680,4 +682,6 @@ config PM_STATS
>         Enable collection of performance management statistics to aid in
>         analyzing and tuning power/performance characteristics of the system
>  
> +source "common/libfdt/Kconfig"
> +
>  endmenu
> diff --git a/xen/common/libfdt/Kconfig b/xen/common/libfdt/Kconfig
> new file mode 100644
> index 000000000000..3abd904b2969
> --- /dev/null
> +++ b/xen/common/libfdt/Kconfig
> @@ -0,0 +1,14 @@
> +config LIBFDT_WIP
> +     bool
> +
> +config LIBFDT_SW
> +    bool
> +
> +config LIBFDT_RW
> +    bool
> +
> +config LIBFDT_EMPTY_TREE
> +    bool
> +
> +config LIBFDT_OVERLAY
> +    bool
> diff --git a/xen/common/libfdt/Makefile b/xen/common/libfdt/Makefile
> index 6ce679f98f47..c832d1849a5c 100644
> --- a/xen/common/libfdt/Makefile
> +++ b/xen/common/libfdt/Makefile
> @@ -1,7 +1,13 @@
> -include $(src)/Makefile.libfdt
>  
>  SECTIONS := text data $(SPECIAL_DATA_SECTIONS)
>  
> +obj-libfdt-y := fdt.o fdt_ro.o

> +obj-libfdt-$(CONFIG_LIBFDT_WIP) += fdt_wip.o
> +obj-libfdt-$(CONFIG_LIBFDT_SW) += fdt_sw.o

... turning these two into:

  obj-libfdt-$(CONFIG_DOMAIN_BUILD_HELPERS) += fdt_sw.o fdt_wip.o

Likewise for fdt_empty_tree.o and fdt_overlay.o for ARM_EFI and OVERLAY_DTB
respectively.

> +obj-libfdt-$(CONFIG_LIBFDT_RW) += fdt_rw.o
> +obj-libfdt-$(CONFIG_LIBFDT_EMPTY_TREE) += fdt_empty_tree.o
> +obj-libfdt-$(CONFIG_LIBFDT_OVERLAY) += fdt_overlay.o
> +
>  # For CONFIG_OVERLAY_DTB, libfdt functionalities will be needed during 
> runtime.
>  ifneq ($(CONFIG_OVERLAY_DTB),y)
>  OBJCOPYFLAGS := $(foreach s,$(SECTIONS),--rename-section .$(s)=.init.$(s))
> @@ -15,7 +21,7 @@ CFLAGS-y += -I$(srctree)/include/xen/libfdt/
>  $(obj)/libfdt.o: $(obj)/libfdt-temp.o FORCE
>       $(call if_changed,objcopy)
>  
> -$(obj)/libfdt-temp.o: $(addprefix $(obj)/,$(LIBFDT_OBJS)) FORCE
> +$(obj)/libfdt-temp.o: $(addprefix $(obj)/,$(obj-libfdt-y)) FORCE
>       $(call if_changed,ld)
>  
> -targets += libfdt-temp.o $(LIBFDT_OBJS)
> +targets += libfdt-temp.o $(obj-libfdt-y)

Cheers,
Alejandro