Re: [PATCH v3 7/9] xen/ppc: Enable bootfdt and boot allocator

[Julien Grall <julien@xxxxxxx>]

Hi Shawn,

On 14/03/2024 22:15, Shawn Anastasio wrote:
> Enable usage of bootfdt for populating the boot info struct from the
> firmware-provided device tree.  Also enable the Xen boot page allocator.
>
> Includes minor changes to bootfdt.c's boot_fdt_info() to tolerate the
> scenario in which the FDT overlaps a reserved memory region, as is the
> case on PPC when booted directly from skiboot.
>
> Signed-off-by: Shawn Anastasio <sanastasio@xxxxxxxxxxxxxxxxxxxxx>
> ---
>   xen/arch/ppc/include/asm/setup.h |  5 +++++
>   xen/arch/ppc/setup.c             | 21 ++++++++++++++++++++-
>   xen/common/device-tree/bootfdt.c | 11 +++++++++--
>   3 files changed, 34 insertions(+), 3 deletions(-)
>
> diff --git a/xen/arch/ppc/include/asm/setup.h b/xen/arch/ppc/include/asm/setup.h
> index 1b2d29c5b6..fe27f61fc3 100644
> --- a/xen/arch/ppc/include/asm/setup.h
> +++ b/xen/arch/ppc/include/asm/setup.h
> @@ -115,4 +115,9 @@ const char *boot_module_kind_as_string(bootmodule_kind 
> kind);
>   struct bootcmdline *boot_cmdline_find_by_kind(bootmodule_kind kind);
>   void populate_boot_allocator(void);
>   
> +/*
> + * bootfdt.c
> + */
> +size_t boot_fdt_info(const void *fdt, paddr_t paddr);
> +
This function is common. So the prototype doesn't belong to a per-arch 
header.
>   #endif /* __ASM_PPC_SETUP_H__ */
> diff --git a/xen/arch/ppc/setup.c b/xen/arch/ppc/setup.c
> index 101bdd8bb6..946167a56f 100644
> --- a/xen/arch/ppc/setup.c
> +++ b/xen/arch/ppc/setup.c
> @@ -1,12 +1,14 @@
>   /* SPDX-License-Identifier: GPL-2.0-or-later */
>   #include <xen/init.h>
>   #include <xen/lib.h>
> +#include <xen/libfdt/libfdt.h>
None of the code below doesn't seem to use libfdt.h directly. So why do 
you need it?
>   #include <xen/mm.h>
>   #include <public/version.h>
>   #include <asm/boot.h>
>   #include <asm/early_printk.h>
>   #include <asm/mm.h>
>   #include <asm/processor.h>
> +#include <asm/setup.h>
>   
>   /* Xen stack for bringing up the first CPU. */
>   unsigned char __initdata cpu0_boot_stack[STACK_SIZE] __aligned(STACK_SIZE);
> @@ -24,6 +26,9 @@ void __init noreturn start_xen(unsigned long r3, unsigned 
> long r4,
>                                  unsigned long r5, unsigned long r6,
>                                  unsigned long r7)
>   {
> +    void *boot_fdt;
boot_fdt is not meant to be modified. So this should be const.

> +    struct bootmodule *xen_bootmodule;
Same here I guess.

> +
>       if ( r5 )
>       {
>           /* Unsupported OpenFirmware boot protocol */
> @@ -32,11 +37,25 @@ void __init noreturn start_xen(unsigned long r3, unsigned 
> long r4,
>       else
>       {
>           /* kexec boot protocol */
> -        boot_opal_init((void *)r3);
> +        boot_fdt = (void *)r3;
> +        boot_opal_init(boot_fdt);
>       }
>   
>       setup_exceptions();
>   
> +    device_tree_flattened = boot_fdt;
> +    boot_fdt_info(boot_fdt, r3);
> +
> +    /*
> +     * Xen relocates itself at the ppc64 entrypoint, so we need to manually 
> mark
> +     * the kernel module.
> +     */
> +    xen_bootmodule = add_boot_module(BOOTMOD_XEN, __pa(_start),
> +                                     PAGE_ALIGN(__pa(_end)), false);
> +    BUG_ON(!xen_bootmodule);
> +
> +    populate_boot_allocator();
> +
>       setup_initial_pagetables();
>   
>       early_printk("Hello, ppc64le!\n");
> diff --git a/xen/common/device-tree/bootfdt.c b/xen/common/device-tree/bootfdt.c
> index 35dbdf3384..1985648b31 100644
> --- a/xen/common/device-tree/bootfdt.c
> +++ b/xen/common/device-tree/bootfdt.c
> @@ -543,12 +543,19 @@ size_t __init boot_fdt_info(const void *fdt, paddr_t 
> paddr)
>       if ( ret < 0 )
>           panic("No valid device tree\n");
>   
> -    add_boot_module(BOOTMOD_FDT, paddr, fdt_totalsize(fdt), false);
> -
>       ret = device_tree_for_each_node(fdt, 0, early_scan_node, NULL);
>       if ( ret )
>           panic("Early FDT parsing failed (%d)\n", ret);
>   
> +    /*
> +     * Add module for the FDT itself after the device tree has been parsed. 
> This
> +     * is required on ppc64le where the device tree passed to Xen may have been
> +     * allocated by skiboot, in which case it will exist within a reserved
> +     * region and this call will fail. This is fine, however, since either way
> +     * the allocator will know not to step on the device tree.
> +     */
> +    add_boot_module(BOOTMOD_FDT, paddr, fdt_totalsize(fdt), false);
I am still a little bit concerned with this change. If there is an 
overlap, then BOOTMOD_FDT will not be created yet Xen will continue. I 
think this needs to be explained in the commit message and in the code 
(maybe on top of BOOTMOD_FDT which should be common). So it will be 
clearer that we cannot rely on BOOTMOD_FDT.
Also, there was some recent discussion for MISRA to check all return or 
use (void) + a comment to explain this is ignored. I think you have part 
of the explanation (see above for the second part), but I would also add 
(void) to make clear this was on purpose.
Cheers,

--
Julien Grall