Re: [PATCH] xen/efi: Handle efi_memmap with Xen relocating itself

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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On Fri, Nov 06, 2026 at 06:18:18PM -0500, Jason Andryuk wrote:
> With Xen booting via Multiboot2/GRUB on an EFI platform, and modified to
> use MULTIBOOT2_LOAD_PREFERENCE_LOW, Xen relocates itself with
> move_xen().
> 
> However a PVH dom0 shows:
> kernel: Failed to lookup header 0x77ffd000 in Xen memory map: error -3
> kernel: ACPI=0x77ffd000
> 
> ... for all the Xen efi memory map lookups.  Dumping the memory map at
> runtime showed it was scrubbed.  Because Xen relocated itself, the
> original physical address referenced through a directmap address goes
> stale.
> 
> Add efi_arch_mmap_buffer_virt() to convert from the initial value to an
> appropriate runtime value.  ARM uses Boot Services AllocatePool(), so it
> must continue to use DIRECTMAP_VIRT_START.
> 
> For x86, we use an offset into the loaded Xen binary at the time of the
> call and add the to XEN_VIRT_START.  ebmalloc() allocates from the Xen
> image memory, so it moves during relocation.  xen_phys_start is set for
> xen.efi in efi_exit_boot(), but not for xen.gz, so it cannot be used
> Use `lea __image_base__(%rip)` to get the current load address, and use
> that to calculate the offset into the Xen image.  That offset is also
> valid from XEN_VIRT_START.
> 
> Fixes: c5b9805bc1f7 ("efi: create new early memory allocator")
> Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>

I did not tested this, but looks correct:
Reviewed-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>

> ---
> I think it could be seen without MULTIBOOT2_LOAD_PREFERENCE_LOW if xen
> chose to relocate itself.  But since grub usually loads xen high, it
> doesn't relocate itself.
> ---
>  xen/arch/arm/efi/efi-boot.h |  5 +++++
>  xen/arch/x86/efi/efi-boot.h | 10 ++++++++++
>  xen/common/efi/boot.c       |  2 +-
>  3 files changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/arch/arm/efi/efi-boot.h b/xen/arch/arm/efi/efi-boot.h
> index 069cc68b0a..717bd8d607 100644
> --- a/xen/arch/arm/efi/efi-boot.h
> +++ b/xen/arch/arm/efi/efi-boot.h
> @@ -449,6 +449,11 @@ static void *__init efi_arch_allocate_mmap_buffer(UINTN map_size)
>      return ptr;
>  }
>  
> +static void *__init efi_arch_mmap_buffer_virt(void *ptr)
> +{
> +    return ptr + DIRECTMAP_VIRT_START;
> +}
> +
>  static void __init efi_arch_edd(void)
>  {
>  }
> diff --git a/xen/arch/x86/efi/efi-boot.h b/xen/arch/x86/efi/efi-boot.h
> index d738b839ee..ffc277ccfd 100644
> --- a/xen/arch/x86/efi/efi-boot.h
> +++ b/xen/arch/x86/efi/efi-boot.h
> @@ -224,6 +224,16 @@ static void *__init efi_arch_allocate_mmap_buffer(UINTN map_size)
>      return ebmalloc(map_size);
>  }
>  
> +static void *__init efi_arch_mmap_buffer_virt(void *ptr)
> +{
> +    unsigned long load_base;
> +
> +    /* RIP-relative address is the current Xen load address. */
> +    asm("lea __image_base__(%%rip), %0" : "=r" (load_base));
> +
> +    return ptr - load_base + XEN_VIRT_START;
> +}
> +
>  static void __init efi_arch_pre_exit_boot(void)
>  {
>      if ( !trampoline_phys )
> diff --git a/xen/common/efi/boot.c b/xen/common/efi/boot.c
> index 89b5831691..5396bad7a5 100644
> --- a/xen/common/efi/boot.c
> +++ b/xen/common/efi/boot.c
> @@ -1425,8 +1425,8 @@ static void __init efi_exit_boot(EFI_HANDLE ImageHandle, EFI_SYSTEM_TABLE *Syste
>      /* Adjust pointers into EFI. */
>      efi_ct = (const void *)efi_ct + DIRECTMAP_VIRT_START;
>      efi_rs = (const void *)efi_rs + DIRECTMAP_VIRT_START;
> -    efi_memmap = (void *)efi_memmap + DIRECTMAP_VIRT_START;
>      efi_fw_vendor = (const void *)efi_fw_vendor + DIRECTMAP_VIRT_START;
> +    efi_memmap = efi_arch_mmap_buffer_virt(efi_memmap);
>  }
>  
>  /* SAF-1-safe */
> -- 
> 2.34.1
> 

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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