[PATCH v2] Use EfiACPIReclaimMemory for ESRT

[Demi Marie Obenour <demi@invisiblethingslab.com>]

A previous patch tried to get Linux to use the ESRT under Xen if it is
in memory of type EfiRuntimeServicesData.  However, this turns out to be
a bad idea.  Ard Biesheuvel pointed out that EfiRuntimeServices* memory
winds up fragmenting both the EFI page tables and the direct map, and
that EfiACPIReclaimMemory is a much better choice for this purpose.

Link: https://lists.xenproject.org/archives/html/xen-devel/2022-09/msg01365.html
Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
---
Changes since v1:

- Add link to Ard Biesheuvel’s post on xen-devel
- Expand comment to mention buggy firmware that place the ESRT in
  EfiBootServicesData.

 xen/common/efi/boot.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/xen/common/efi/boot.c b/xen/common/efi/boot.c
index db0340c8e2628314226c618dda11ede4c62fdf3b..b3de1011ee58a67a82a94da050eb1343f4e37faa 100644
--- a/xen/common/efi/boot.c
+++ b/xen/common/efi/boot.c
@@ -601,11 +601,14 @@ static size_t __init get_esrt_size(const EFI_MEMORY_DESCRIPTOR *desc)
     if ( physical_start > esrt || esrt - physical_start >= len )
         return 0;
     /*
-     * The specification requires EfiBootServicesData, but accept
-     * EfiRuntimeServicesData, which is a more logical choice.
+     * The specification requires EfiBootServicesData, but also accept
+     * EfiRuntimeServicesData (for compatibility with buggy firmware)
+     * and EfiACPIReclaimMemory (which will contain the tables after
+     * successful kexec).
      */
     if ( (desc->Type != EfiRuntimeServicesData) &&
-         (desc->Type != EfiBootServicesData) )
+         (desc->Type != EfiBootServicesData) &&
+         (desc->Type != EfiACPIReclaimMemory) )
         return 0;
     available_len = len - (esrt - physical_start);
     if ( available_len <= offsetof(EFI_SYSTEM_RESOURCE_TABLE, Entries) )
@@ -1144,18 +1147,19 @@ static void __init efi_relocate_esrt(EFI_SYSTEM_TABLE *SystemTable)
     for ( i = 0; i < info_size; i += mdesc_size )
     {
         /*
-         * ESRT needs to be moved to memory of type EfiRuntimeServicesData
+         * ESRT needs to be moved to memory of type EfiACPIReclaimMemory
          * so that the memory it is in will not be used for other purposes.
          */
         void *new_esrt = NULL;
-        size_t esrt_size = get_esrt_size(memory_map + i);
+        const EFI_MEMORY_DESCRIPTOR *desc = memory_map + i;
+        size_t esrt_size = get_esrt_size(desc);
 
         if ( !esrt_size )
             continue;
-        if ( ((EFI_MEMORY_DESCRIPTOR *)(memory_map + i))->Type ==
-             EfiRuntimeServicesData )
+        if ( desc->Type == EfiRuntimeServicesData ||
+             desc->Type == EfiACPIReclaimMemory )
             break; /* ESRT already safe from reuse */
-        status = efi_bs->AllocatePool(EfiRuntimeServicesData, esrt_size,
+        status = efi_bs->AllocatePool(EfiACPIReclaimMemory, esrt_size,
                                       &new_esrt);
         if ( status == EFI_SUCCESS && new_esrt )
         {
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab