Re: [PATCH v2 1/2] kexec: Add KEXEC_FILE_NO_CMA as a legal flag

[Brian Mak <makb@juniper.net>]

On Aug 21, 2025, at 8:33 PM, Baoquan He <bhe@redhat.com> wrote:

> Yeah, this is a good catch and great fix. Without this fix,
> kexec_file_load syscall will failed and return '-EINVAL' when
> KEXEC_FILE_NO_CMA is specified just as below code shows. So, for this
> patch,
> 
> Acked-by: Baoquan He <bhe@redhat.com>

Hi Baoquan,

Thanks for the ACK!

> And, by the way, has the user space kexec-tools got the change merged
> to allow KEXEC_FILE_NO_CMA specified?

I don't see any recent commits to kexec-tools to support
KEXEC_FILE_NO_CMA.

>> From: Brian Mak <makb@juniper.net>
>> Subject: x86/kexec: carry forward the boot DTB on kexec
>> Date: Tue, 5 Aug 2025 14:15:27 -0700
>> 
>> Currently, the kexec_file_load syscall on x86 does not support passing a
>> device tree blob to the new kernel.  Some embedded x86 systems use device
>> trees.  On these systems, failing to pass a device tree to the new kernel
>> causes a boot failure.
>> 
>> To add support for this, we copy the behavior of ARM64 and PowerPC and
>> copy the current boot's device tree blob for use in the new kernel.  We do
>> this on x86 by passing the device tree blob as a setup_data entry in
>> accordance with the x86 boot protocol.
>> 
>> This behavior is gated behind the KEXEC_FILE_FORCE_DTB flag.
>> 
>> Link: https://urldefense.com/v3/__https://lkml.kernel.org/r/20250805211527.122367-3-makb@juniper.net__;!!NEt6yMaO-gk!EbJyF8xO2E51MyYdN3_zqCBBMj0JKoiKoPuG_8vEctQMk9uCyjX0LdSEH_FGkPDV8egxzc7w$
>> Signed-off-by: Brian Mak <makb@juniper.net>
>> Cc: Alexander Graf <graf@amazon.com>
>> Cc: Baoquan He <bhe@redhat.com>
>> Cc: Borislav Betkov <bp@alien8.de>
>> Cc: Dave Young <dyoung@redhat.com>
>> Cc: "H. Peter Anvin" <hpa@zytor.com>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: Rob Herring <robh@kernel.org>
>> Cc: Saravana Kannan <saravanak@google.com>
>> Cc: Thomas Gleinxer <tglx@linutronix.de>
>> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
>> ---
>> 
>> arch/x86/kernel/kexec-bzimage64.c |   47 ++++++++++++++++++++++++++--
>> include/linux/kexec.h             |    5 ++
>> include/uapi/linux/kexec.h        |    4 ++
>> kernel/kexec_file.c               |    1
>> 4 files changed, 53 insertions(+), 4 deletions(-)
>> 
>> --- a/arch/x86/kernel/kexec-bzimage64.c~x86-kexec-carry-forward-the-boot-dtb-on-kexec
>> +++ a/arch/x86/kernel/kexec-bzimage64.c
>> @@ -16,6 +16,8 @@
>> #include <linux/kexec.h>
>> #include <linux/kernel.h>
>> #include <linux/mm.h>
>> +#include <linux/libfdt.h>
>> +#include <linux/of_fdt.h>
>> #include <linux/efi.h>
>> #include <linux/random.h>
>> 
>> @@ -212,6 +214,28 @@ setup_efi_state(struct boot_params *para
>> }
>> #endif /* CONFIG_EFI */
>> 
>> +#ifdef CONFIG_OF_FLATTREE
>> +static void setup_dtb(struct boot_params *params,
>> +                   unsigned long params_load_addr,
>> +                   unsigned int dtb_setup_data_offset)
>> +{
>> +     struct setup_data *sd = (void *)params + dtb_setup_data_offset;
>> +     unsigned long setup_data_phys, dtb_len;
>> +
>> +     dtb_len = fdt_totalsize(initial_boot_params);
>> +     sd->type = SETUP_DTB;
>> +     sd->len = dtb_len;
>> +
>> +     /* Carry over current boot DTB with setup_data */
>> +     memcpy(sd->data, initial_boot_params, dtb_len);
>> +
>> +     /* Add setup data */
>> +     setup_data_phys = params_load_addr + dtb_setup_data_offset;
>> +     sd->next = params->hdr.setup_data;
>> +     params->hdr.setup_data = setup_data_phys;
>> +}
>> +#endif /* CONFIG_OF_FLATTREE */
>> +
>> static void
>> setup_ima_state(const struct kimage *image, struct boot_params *params,
>>              unsigned long params_load_addr,
>> @@ -336,6 +360,17 @@ setup_boot_parameters(struct kimage *ima
>>                      sizeof(struct efi_setup_data);
>> #endif
>> 
>> +#ifdef CONFIG_OF_FLATTREE
>> +     if (image->force_dtb && initial_boot_params) {
>> +             setup_dtb(params, params_load_addr, setup_data_offset);
>> +             setup_data_offset += sizeof(struct setup_data) +
>> +                                  fdt_totalsize(initial_boot_params);
>> +     } else {
>> +             pr_debug("Not carrying over DTB, force_dtb = %d\n",
>> +                      image->force_dtb);
>> +     }
>> +#endif
>> +
>>      if (IS_ENABLED(CONFIG_IMA_KEXEC)) {
>>              /* Setup IMA log buffer state */
>>              setup_ima_state(image, params, params_load_addr,
>> @@ -529,6 +564,12 @@ static void *bzImage64_load(struct kimag
>>                              sizeof(struct setup_data) +
>>                              RNG_SEED_LENGTH;
>> 
>> +#ifdef CONFIG_OF_FLATTREE
>> +     if (image->force_dtb && initial_boot_params)
>> +             kbuf.bufsz += sizeof(struct setup_data) +
>> +                           fdt_totalsize(initial_boot_params);
>> +#endif
>> +
>>      if (IS_ENABLED(CONFIG_IMA_KEXEC))
>>              kbuf.bufsz += sizeof(struct setup_data) +
>>                            sizeof(struct ima_setup_data);
>> @@ -537,7 +578,7 @@ static void *bzImage64_load(struct kimag
>>              kbuf.bufsz += sizeof(struct setup_data) +
>>                            sizeof(struct kho_data);
>> 
>> -     params = kzalloc(kbuf.bufsz, GFP_KERNEL);
>> +     params = kvzalloc(kbuf.bufsz, GFP_KERNEL);
> 
> Wondering how big the dtb blob is, can you explain a little bit about
> the kvzalloc usage here?
> 
> Except of this, I have no other concern about this patch.
> 
> And what's your plan about the user space kexec-tool change?

When I tested this earlier on x86, the DTB was allowed to be up to just
under 64 pages large before the DTB failed to load. This is because it
has to fit into an early_memremap() mapping (relevant code snippet at
the bottom). Since the allocation can be many pages, I changed the
kzalloc to a kvzalloc.

For the kexec-tools change, I have a draft change that I've already
shared on this thread for testing purposes. I believe you said you were
going to test it, but I haven't heard anything back from that yet. I'll
raise that change for review properly once this kernel commit is in
mainline.

---------

void __init x86_flattree_get_config(void)
{
#ifdef CONFIG_OF_EARLY_FLATTREE
	u32 size, map_len;
	void *dt;

	if (initial_dtb) {
		map_len = max(PAGE_SIZE - (initial_dtb & ~PAGE_MASK), (u64)128);

		dt = early_memremap(initial_dtb, map_len);
		size = fdt_totalsize(dt);
		if (map_len < size) {
			early_memunmap(dt, map_len);
			dt = early_memremap(initial_dtb, size);
			map_len = size;
		}

		early_init_dt_verify(dt, __pa(dt));
	}

	unflatten_and_copy_device_tree();

	if (initial_dtb)
		early_memunmap(dt, map_len);
#endif
	if (acpi_disabled && of_have_populated_dt())
		x86_init.mpparse.parse_smp_cfg = x86_dtb_parse_smp_config;
}

---------

Thanks,
Brian