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

["Alejandro Vallejo" <alejandro.vallejo@xxxxxxxxx>]

On Thu Mar 13, 2025 at 1:14 PM GMT, Jan Beulich wrote:
> 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).

I feel the rationale is the same on both paths, so the comment blocks ought to
be aligned (whichever way). But I have no strong motivations and would be fine
dropping the middle paragraph here.

If that's your only remark, I'm happy for it to be dropped on commit.

>
> 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

ACPI tables are populated by hvmloader, while libxl generates those of PVH.

dom->acpi_modules are populated by libxl__dom_load_acpi(), which is gated on
the type being PVH (see the caller of this function). So this loop should be
effectively skipped.

I called it the PVH path because it happens to be at the moment. Nothing
prevents this path from being the HVM path too, but that involves rewiring
hvmloader.

Cheers,
Alejandro