Re: [PATCH] xen/arm: acpi: Support memory reserve configuration table

[Julien Grall <julien@xxxxxxx>]

Hi Leo,

On 18/08/2022 08:34, Leo Yan wrote:
> On Wed, Aug 17, 2022 at 03:17:53PM +0200, Jan Beulich wrote:
> > Furthermore - what if Linux decided to change their structure? Or
> > is there a guarantee that they won't? Generally such structures
> > belong in the public interface, guaranteeing forward compatibility
> > even if Linux decided to change / extend theirs (at which point
> > consuming code there would need to do translation, but maybe using
> > a Xen-defined struct [plus translation in Linux] right away would
> > be best).
> I saw Ard has helped to answer this question in his email.  As Ard
> said, the general way is to rely on Linux EFI stub to allocate the
> data structure for MEMRESERVE configuration table.
>
> Given Xen uses pseudo EFI booting (the ACPI table is passed via DT), in
> short term I don't think Xen can support Linux EFI stub,
I agree that using Linux EFI stub will require more effort. I would also 
need to go through the mailing list archives (or maybe Stefano 
remember?) to understand why we decided against using the EFI stub.
> so we need to
> maintain this structure in Xen as well.
I have looked at the commit message. IIUC, if this table is not created 
then Kexec will not work. Is there anything else that would not work?
IOW, would Linux be unusable if we don't create MEMRESERVE?

> This structure eventually will not change frequently, so I assume
> later we will have little effort for maintainence it
The problem is not really "maintainance" here. It is more that if Linux 
is updating the structure in a non-backward compatible way, then new
version of Linux would not boot on older Xen.

We would have similar probable with new Xen booting older Xen because 
the hypervisor doesn't know (and should not need to know) which OS is used.
In the nutshells, Xen should only use stable interface. If MEMRESERVE is 
really necessary then we should either switch to the Linux EFI stub or 
standardize MEMRESERVE.
Cheers,

--
Julien Grall