Re: [RFC PATCH] xen/device-tree: Switch back to dt_unreserved_regions() in boot allocator

["Orzel, Michal" <michal.orzel@xxxxxxx>]

On 24/03/2025 22:27, Oleksandr Tyshchenko wrote:
> 
> 
> On the device-tree-based Arm64 system, if Xen is built with
> CONFIG_ACPI=y, CONFIG_STATIC_MEMORY=y, and the static memory range
> is provided in the host device tree, the BUG is triggered in
> common/page_alloc.c during Xen's early boot. The BUG occurs when
> the first page from the static range is fed to the domain
> sub-allocator and finally ends up in mark_page_free().
> The pg->count_info & PGC_state is not in the state that
> the code expects to see there.
> 
> The problem is that the static range gets mistakenly unreserved
> in populate_boot_allocator() and reaches init_boot_pages().
> This happens since by the time the populate_boot_allocator()
> is executed, the evaluated in fw_unreserved_regions()
> an acpi_disabled variable is still false (CONFIG_ACPI=y) and
> dt_unreserved_regions() which should simply skip that static range
> does not get called. The acpi_disabled will be set to the actual
> value later on in acpi_boot_table_init().
> 
> To avoid unreserving the whole region (including potential reserved
> memory ranges such as static memory) open code fw_unreserved_regions()
> and add a comment.
> 
> Another solution could be to call acpi_boot_table_init() before
> setup_mm() in Arm64's start_xen().
Interesting issue. However IMO the problem is located somewhere else. At the
moment, with CONFIG_ACPI=n, acpi_disabled is set to true. With CONFIG_ACPI=y,
it's set to false even though entire acpi_boot_table_init() is written with
assumption that ACPI is off by default at the start. That's why
acpi_boot_table_init() calls e.g. disable_acpi() if it finds that user specified
acpi=off on cmdline. Why would it do that if the assumption was that
acpi_disabled is false? There's even a comment:
https://xenbits.xenproject.org/gitweb/?p=xen.git;a=blob;f=xen/arch/arm/acpi/boot.c;hb=HEAD#l257
that clearly says that "at this point ACPI is disabled".

Therefore, I think the fix should look as follows:
diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
index ffcae900d72e..9e94f1a8c761 100644
--- a/xen/arch/arm/setup.c
+++ b/xen/arch/arm/setup.c
@@ -57,7 +57,7 @@
 struct cpuinfo_arm __read_mostly system_cpuinfo;

 #ifdef CONFIG_ACPI
-bool __read_mostly acpi_disabled;
+bool __read_mostly acpi_disabled = true;
 #endif

 domid_t __read_mostly max_init_domid;

~Michal