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

["Alejandro Vallejo" <alejandro.vallejo@cloud.com>]

On Tue Mar 11, 2025 at 8:30 AM GMT, Jan Beulich wrote:
> 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@cloud.com>
> > ---
> > 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.
>

Do you mean ACPI hotplug? I don't think I've ever seen a real system with
support for it. I don't suppose you remember any specifically? I'd be quite
interested to have a look at their ACPI dumps.

> > 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.

Fair enough. Let me send a v2 with that.

>
> Jan

Cheers,
Alejandro