Re: [PATCH 5/5] xen: tolerate ACPI NVS memory overlapping with Xen allocated memory

[Jan Beulich <jbeulich@suse.com>]

On 07.08.2024 14:05, Jan Beulich wrote:
> On 07.08.2024 12:41, Juergen Gross wrote:
>> In order to minimize required special handling for running as Xen PV
>> dom0, the memory layout is modified to match that of the host. This
>> requires to have only RAM at the locations where Xen allocated memory
>> is living. Unfortunately there seem to be some machines, where ACPI
>> NVS is located at 64 MB, resulting in a conflict with the loaded
>> kernel or the initial page tables built by Xen.
>>
>> As ACPI NVS needs to be accessed by the kernel only for saving and
>> restoring it across suspend operations, it can be relocated in the
>> dom0's memory map by swapping it with unused RAM (this is possible
>> via modification of the dom0 P2M map).
> 
> While the kernel may not (directly) need to access it for other purposes,
> what about AML accessing it? As you can't advertise the movement to ACPI,
> and as non-RAM mappings are carried out by drivers/acpi/osl.c:acpi_map()
> using acpi_os_ioremap(), phys-to-machine translations won't cover for
> that (unless I'm overlooking something, which unfortunately seems like I
> might be).

Thinking some more about this, the above may be pointing in the wrong
direction. If from acpi_os_ioremap() downwards no address translation
(PFN -> MFN) occurred, then what's coming from AML would still be
handled correctly as far as page table entries go. The problem then
might instead be that the mapping would appear to be covering RAM, not
the ACPI NVS region (and there may be checks for that).

Jan