Re: [PATCH v6 04/22] kho: return virtual address of mem_map from kho_get_mem_map()

On Sat, Aug 01, 2026 at 10:48:13AM +0200, Pratyush Yadav wrote:
> From: "Pratyush Yadav (Google)" <[email protected]>
> 
> Currently the preserved memory map address is returned by
> kho_get_mem_map_phys(). It is only used by kho_populate().
> kho_populate() doesn't use the actual value. It only cares that the
> address exists and is valid.
> 
> In coming patches, more callers will be added, all of which will need
> the virtual address of the preserved memory map. Since kho_populate()
> doesn't care about the actual value and only cares about validity, it
> can also use the virtual address returned by kho_get_mem_map(). It only
> needs to make sure the returned value is not NULL.
> 
> Rename kho_get_mem_map_phys() to kho_get_mem_map() and return the
> virtual address of the preserved memory map.
> 
> Signed-off-by: Pratyush Yadav (Google) <[email protected]>
> ---
>  kernel/liveupdate/kexec_handover.c | 25 +++++++++++++++++--------
>  1 file changed, 17 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index e7451743b87e..5c35c11c273b 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -512,19 +512,24 @@ static int __init kho_preserved_memory_reserve(unsigned long key)
>  	return 0;
>  }
>  
> -/* Returns physical address of the preserved memory map from FDT */
> -static phys_addr_t __init kho_get_mem_map_phys(const void *fdt)
> +/* Returns virtual address of the preserved memory map from FDT */
> +static __init void *kho_get_mem_map(const void *fdt)
>  {
>  	const void *mem_ptr;
> +	phys_addr_t mem_map_phys;
>  	int len;
>  
>  	mem_ptr = fdt_getprop(fdt, 0, KHO_FDT_MEMORY_MAP_PROP_NAME, &len);
>  	if (!mem_ptr || len != sizeof(u64)) {
>  		pr_err("failed to get preserved memory map\n");
> -		return 0;
> +		return NULL;
>  	}
>  
> -	return get_unaligned((const u64 *)mem_ptr);
> +	mem_map_phys = get_unaligned((const u64 *)mem_ptr);
> +	if (!mem_map_phys)
> +		return NULL;
> +
> +	return phys_to_virt(mem_map_phys);

This time sashiko found a real issue with this on arm64:

  Will calling phys_to_virt() unconditionally here cause a panic on ARM64
  during early boot?

  When booting with KHO properties, early_init_dt_scan() calls kho_populate()
  which then calls kho_get_mem_map(). Since this happens before
  arm64_memblock_init() sets up memstart_addr, the internal phys_to_virt()
  implementation evaluates PHYS_OFFSET with an uninitialized memstart_addr.
  This fails the VM_BUG_ON(memstart_addr & 1) assertion when CONFIG_DEBUG_VM
  is enabled.

I fixed this up by keeping kho_get_mem_map_phys() and adding a thin
kho_get_mem_map() that returns the virtual address.

This caused some rebase conflicts afterwards, please check I didn't mess up
anything :)

>  }
>  
>  /*
> @@ -1647,9 +1652,8 @@ void __init kho_populate(phys_addr_t fdt_phys, u64 fdt_len,
>  {
>  	unsigned int scratch_cnt = scratch_len / sizeof(*kho_scratch);
>  	struct kho_scratch *scratch = NULL;
> -	phys_addr_t mem_map_phys;
> -	void *fdt = NULL;
>  	bool populated = false;
> +	void *fdt = NULL;
>  	int err;
>  
>  	/* Validate the input FDT */
> @@ -1671,8 +1675,13 @@ void __init kho_populate(phys_addr_t fdt_phys, u64 fdt_len,
>  		goto unmap_fdt;
>  	}
>  
> -	mem_map_phys = kho_get_mem_map_phys(fdt);
> -	if (!mem_map_phys)
> +	/*
> +	 * At this point phys_to_virt() doesn't work properly and so
> +	 * kho_get_mem_map() can return a pre-KASLR virtual address. But here we
> +	 * only want to make sure the mem_map is valid so the actual value
> +	 * doesn't matter as long as it isn't NULL.
> +	 */
> +	if (!kho_get_mem_map(fdt))
>  		goto unmap_fdt;
>  
>  	scratch = early_memremap(scratch_phys, scratch_len);
> -- 
> 2.55.0.571.g244d577d93-goog
> 

-- 
Sincerely yours,
Mike.