Re: [PATCH] xen/arm: Remove EXPERT dependancy

[Julien Grall <julien@xxxxxxx>]

Hi Stefano,

On 22/10/2020 22:17, Stefano Stabellini wrote:
> On Thu, 22 Oct 2020, Julien Grall wrote:
> > On 22/10/2020 02:43, Elliott Mitchell wrote:
> > > Linux requires UEFI support to be enabled on ARM64 devices.  While many
> > > ARM64 devices lack ACPI, the writing seems to be on the wall of UEFI/ACPI
> > > potentially taking over.  Some common devices may need ACPI table
> > > support.
> > >
> > > Presently I think it is worth removing the dependancy on CONFIG_EXPERT.
> > The idea behind EXPERT is to gate any feature that is not considered to be
> > stable/complete enough to be used in production.
> Yes, and from that point of view I don't think we want to remove EXPERT
> from ACPI yet. However, the idea of hiding things behind EXPERT works
> very well for new esoteric features, something like memory introspection
> or memory overcommit.
Memaccess is not very new ;).

> It does not work well for things that are actually
> required to boot on the platform.
I am not sure where is the problem. It is easy to select EXPERT from the 
menuconfig. It also hints the user that the feature may not fully work.
> Typically ACPI systems don't come with device tree at all (RPi4 being an
> exception), so users don't really have much of a choice in the matter.
And they typically have IOMMUs.

>  From that point of view, it would be better to remove EXPERT from ACPI,
> maybe even build ACPI by default, *but* to add a warning at boot saying
> something like:
>
> "ACPI support is experimental. Boot using Device Tree if you can."
>
>
> That would better convey the risks of using ACPI, while at the same time
> making it a bit easier for users to boot on their ACPI-only platforms.
Right, I agree that this make easier for users to boot Xen on ACPI-only 
platform. However, based on above, it is easy enough for a developper to 
rebuild Xen with ACPI and EXPERT enabled.
So what sort of users are you targeting?

I am sort of okay to remove EXPERT. But I still think building ACPI by 
default is still wrong because our default .config is meant to be 
(security) supported. I don't think ACPI can earn this qualification today.
In order to remove EXPERT, there are a few things to needs to be done 
(or checked):
    1) SUPPORT.MD has a statement about ACPI on Arm
    2) DT is favored over ACPI if the two firmware tables are present.

> > I don't consider the ACPI complete because the parsing of the IORT (used to
> > discover SMMU and GICv3 ITS) is not there yet.
> >
> > I vaguely remember some issues on system using SMMU (e.g. Thunder-X) because
> > Dom0 will try to use the IOMMU and this would break PV drivers.
> I am not sure why Dom0 using the IOMMU would break PV drivers? Is it
> because the pagetable is not properly updated when mapping foreign
> pages?
IIRC, yes. This would need to be tested again.

Cheers,

--
Julien Grall