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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.03.2025 16:25, 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>
> ---
> I really, really, really dislike this idea of accessing the MADT from
> AML.

I think this isn't Xen's invention, but something I've seen in various
systems' AML.

> In time I'll try to implement something to stop doing it, but for
> the time being I find it preferable to align libxl to hvmloader rather
> than trying to restrict what's reclaimable and what isn't.
> ---
>  tools/firmware/hvmloader/e820.c | 4 ++++
>  tools/libs/light/libxl_x86.c    | 2 +-
>  2 files changed, 5 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..265da8072a59 100644
> --- a/tools/libs/light/libxl_x86.c
> +++ b/tools/libs/light/libxl_x86.c
> @@ -742,7 +742,7 @@ static int domain_construct_memmap(libxl__gc *gc,
>              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;

This likely needs a comment then, too.

Jan