Re: [PATCH v4] hvmloader: indicate ACPI tables with "ACPI data" type in e820

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.09.2020 17:41, Igor Druzhinin wrote:
> Guest kernel does need to know in some cases where the tables are located
> to treat these regions properly. One example is kexec process where
> the first kernel needs to pass ACPI region locations to the second
> kernel which is now a requirement in Linux after 02a3e3cdb7f12 ("x86/boot:
> Parse SRAT table and count immovable memory regions") in order for kexec
> transition to actually work.
> 
> That commit introduced accesses to XSDT and SRAT while the second kernel
> is still using kexec transition tables. The transition tables do not have
> e820 "reserved" regions mapped where those tables are located currently
> in a Xen guest. Instead "ACPI data" regions are mapped with the transition
> tables that was introduced by the following commit 6bbeb276b7 ("x86/kexec:
> Add the EFI system tables and ACPI tables to the ident map").
> 
> Reserve 1MB (out of 16MB currently available) right after ACPI info page for
> ACPI tables exclusively but populate this region on demand and only indicate
> populated memory as "ACPI data" since according to ACPI spec that memory is
> reclaimable by the guest if necessary. That is close to how we treat
> the same ACPI data in PVH guests. 1MB should be enough for now but could be
> later extended if required.
> 
> Signed-off-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>

After committing this I'm now somewhat uncertain whether to queue this
for the stable trees. Does either of you (or anyone else) have any clear
opinion either way?

Jan