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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 11.03.2025 10:29, Alejandro Vallejo wrote:
> 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 <alejandro.vallejo@xxxxxxxxx>
> ---
> 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(-)
> 
> 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.
> +     */

When asking for a comment here I really only was after what the last paragraph 
says.
Especially the middle paragraph seems questionable to me: It would not only be 
ACPI-
unawareness, but also E820-unawareness, for the range to be prematurely 
re-used. And
buggy bootloaders really would need fixing, I think - they'd put OSes into 
trouble on
real hardware as well.

In short - I'd like to ask that the middle paragraph be dropped from here (which
surely could be done while committing).

However, there's a second concern: You say "PVH" in the title, yet this 
function is
in use also for HVM, and ...

>      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++;
>          }
>      }

... this code is outside of any conditionals. This imo needs sorting one way or
another.

Jan