Re: [PATCH v4 1/2] Avoid using EFI tables Xen may have clobbered

From: Jan Beulich
Date: Wed Oct 05 2022 - 02:20:42 EST

On 04.10.2022 17:46, Demi Marie Obenour wrote:
> Linux has a function called efi_mem_reserve() that is used to reserve
> EfiBootServicesData memory that contains e.g. EFI configuration tables.
> This function does not work under Xen because Xen could have already
> clobbered the memory. efi_mem_reserve() not working is the whole reason
> for this thread, as it prevents EFI tables that are in
> EfiBootServicesData from being used under Xen.
> A much nicer approach would be for Xen to reserve boot services memory
> unconditionally, but provide a hypercall that dom0 could used to free
> the parts of EfiBootServicesData memory that are no longer needed. This
> would allow efi_mem_reserve() to work normally.

efi_mem_reserve() actually working would be a layering violation;
controlling the EFI memory map is entirely Xen's job.

As to the hypercall you suggest - I wouldn't mind its addition, but only
for the case when -mapbs is used. As I've indicated before, I'm of the
opinion that default behavior should be matching the intentions of the
spec, and the intention of EfiBootServices* is for the space to be
reclaimed. Plus I'm sure you realize there's a caveat with Dom0 using
that hypercall: It might use it for regions where data lives which it
wouldn't care about itself, but which an eventual kexec-ed (or alike)
entity would later want to consume. Code/data potentially usable by
_anyone_ between two resets of the system cannot legitimately be freed
(and hence imo is wrong to live in EfiBootServices* regions). In a way
one could view the Dom0 kernel as an "or alike" entity ...