[v2] tools: Mark ACPI SDTs as NVS in the PVH build path

Commit Message

Alejandro Vallejo March 11, 2025, 9:29 a.m. UTC

Commit cefeffc7e583 marked ACPI tables as NVS in the hvmloader path
because SeaBIOS may otherwise just mark it as RAM. There is, however,
yet another reason to do it even in the PVH path. Xen's incarnation of
AML relies on having access to some ACPI tables (e.g: _STA of Processor
objects relies on reading the processor online bit in its MADT entry)

This is problematic if the OS tries to reclaim ACPI memory for page
tables as it's needed for runtime and can't be reclaimed after the OSPM
is up and running.

Fixes: de6d188a519f("hvmloader: flip "ACPI data" to "ACPI NVS" type for ACPI table region)"
Signed-off-by: Alejandro Vallejo <[e-mail]>
---
v1->v2:
  * Copy explanatory comment in hvmloader/e820.c to its libxl_x86.c counterpart

---
 tools/firmware/hvmloader/e820.c |  4 ++++
 tools/libs/light/libxl_x86.c    | 17 ++++++++++++++++-
 2 files changed, 20 insertions(+), 1 deletion(-)

Patch

diff --git a/tools/firmware/hvmloader/e820.c b/tools/firmware/hvmloader/e820.c
index c490a0bc790c..86d39544e887 100644
--- a/tools/firmware/hvmloader/e820.c
+++ b/tools/firmware/hvmloader/e820.c
@@ -210,6 +210,10 @@  int build_e820_table(struct e820entry *e820,
      * space reuse by an ACPI unaware / buggy bootloader, option ROM, etc.
      * before an ACPI OS takes control. This is possible due to the fact that
      * ACPI NVS memory is explicitly described as non-reclaimable in ACPI spec.
+     *
+     * Furthermore, Xen relies on accessing ACPI tables from within the AML
+     * code exposed to guests. So Xen's ACPI tables are not, in general,
+     * reclaimable.
      */
 
     if ( acpi_enabled )
diff --git a/tools/libs/light/libxl_x86.c b/tools/libs/light/libxl_x86.c
index a3164a3077fe..2ba96d12e595 100644
--- a/tools/libs/light/libxl_x86.c
+++ b/tools/libs/light/libxl_x86.c
@@ -737,12 +737,27 @@  static int domain_construct_memmap(libxl__gc *gc,
         nr++;
     }
 
+    /*
+     * Mark populated reserved memory that contains ACPI tables as ACPI NVS.
+     * That should help the guest to treat it correctly later: e.g. pass to
+     * the next kernel on kexec.
+     *
+     * Using NVS type instead of a regular one helps to prevent potential
+     * space reuse by an ACPI unaware / buggy bootloader, option ROM, etc.
+     * before an ACPI OS takes control. This is possible due to the fact that
+     * ACPI NVS memory is explicitly described as non-reclaimable in ACPI spec.
+     *
+     * Furthermore, Xen relies on accessing ACPI tables from within the AML
+     * code exposed to guests. So Xen's ACPI tables are not, in general,
+     * reclaimable.
+     */
+
     for (i = 0; i < MAX_ACPI_MODULES; i++) {
         if (dom->acpi_modules[i].length) {
             e820[nr].addr = dom->acpi_modules[i].guest_addr_out & ~(page_size - 1);
             e820[nr].size = dom->acpi_modules[i].length +
                 (dom->acpi_modules[i].guest_addr_out & (page_size - 1));
-            e820[nr].type = E820_ACPI;
+            e820[nr].type = E820_NVS;
             nr++;
         }
     }