Re: [Xen-devel] [PATCH] efi/libstub/fdt: Standardize the names of EFI stub parameters

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Fri, Sep 11, 2015 at 03:30:15PM +0200, Ard Biesheuvel wrote:
> On 11 September 2015 at 15:14, Stefano Stabellini
> <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> > On Fri, 11 Sep 2015, Daniel Kiper wrote:
> >> On Thu, Sep 10, 2015 at 05:23:02PM +0100, Mark Rutland wrote:
> >> > > > C) When you could go:
> >> > > >
> >> > > >    DT -> Discover Xen -> Xen-specific stuff -> Xen-specific EFI/ACPI 
> >> > > > discovery
> >> > >
> >> > > I take you mean discovering Xen with the usual Xen hypervisor node on
> >> > > device tree. I think that C) is a good option actually. I like it. Not
> >> > > sure why we didn't think about this earlier. Is there anything EFI or
> >> > > ACPI which is needed before Xen support is discovered by
> >> > > arch/arm64/kernel/setup.c:setup_arch -> xen_early_init()?
> >> >
> >> > Currently lots (including the memory map). With the stuff to support
> >> > SPCR, the ACPI discovery would be moved before xen_early_init().
> >> >
> >> > > If not, we could just go for this. A lot of complexity would go away.
> >> >
> >> > I suspect this would still be fairly complex, but would at least prevent
> >> > the Xen-specific EFI handling from adversely affecting the native case.
> >> >
> >> > > > D) If you want to be generic:
> >> > > >    EFI -> EFI application -> EFI tables -> ACPI tables -> 
> >> > > > Xen-specific stuff
> >> > > >           \------------------------------------------/
> >> > > >            (virtualize these, provide shims to Dom0, but handle
> >> > > >             everything in Xen itself)
> >> > >
> >> > > I think that this is good in theory but could turn out to be a lot of
> >> > > work in practice. We could probably virtualize the RuntimeServices but
> >> > > the BootServices are troublesome.
> >> >
> >> > What's troublesome with the boot services?
> >> >
> >> > What can't be simulated?
> >>
> >> How do you want to access bare metal EFI boot services from dom0 if they
> >> were shutdown long time ago before loading dom0 image? What do you need
> >> from EFI boot services in dom0?
> >
> > That's right. Trying to emulate BootServices after the real
> > ExitBootServices has already been called seems like a very bad plan.
> >
> > I think that whatever interface we come up with, would need to be past
> > ExitBootServices.
>
> It feels like this discussion is going in circles.
>
> When we discussed this six months ago, we already concluded that,
> since UEFI is the only specified way that the presence of ACPI is
> advertised on an ARM system, we need to emulate UEFI to some extent.
>
> So we need the EFI system table to expose the UEFI configuration table
> that carries the ACPI root pointer.
>
> Since ACPI support also relies on the UEFI memory map (I think?), we
> need that as well.
>
> These two items are exactly what we pass via the UEFI DT properties,
> so we should indeed promote the current de-facto binding to a proper
> binding, and renaming the properties makes sense in that context.
>
> I agree that this should also include a description of the expected
> state of the firmware, i.e., that ExitBootServices() has been called,
> and that the memory map has been populated with virtual address, which
> have been installed using SetVirtualAddressMap() if they differ from
> the physical addresses. (The current implementation on the kernel side
> is perfectly capable of dealing with a 1:1 mapping).
>
> Beyond that, there is no point in pretending to be a full UEFI
> implementation, imo. Boot services are not required, nor are runtime
> services (only the current EFI init code on arm needs to be modified
> to deal with a NULL runtime services pointer)

Taking into account above I think that you have most of the code in place.
Please take a look at linux/arch/x86/xen/efi.c, linux/drivers/acpi/osl.c
and linux/drivers/xen/efi.c (maybe somewhere else). In general you should
create ARM version of xen_efi_init() (x86 version you can find in
linux/drivers/xen/efi.c; it is very simple thing), maybe add some
code in a few places and voila.

Daniel

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