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

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Mon, Sep 14, 2015 at 11:43:27AM +0200, Ard Biesheuvel wrote:
> On 14 September 2015 at 11:25, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> > On Sat, Sep 12, 2015 at 12:36:55PM +0100, Daniel Kiper wrote:
> >> On Fri, Sep 11, 2015 at 05:25:59PM +0100, Mark Rutland wrote:
> >> > On Fri, Sep 11, 2015 at 01:46:43PM +0100, Daniel Kiper wrote:
> >> > > On Thu, Sep 10, 2015 at 05:23:02PM +0100, Mark Rutland wrote:
> >>
> >> [...]
> >>
> >> > > > 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?
> >> >
> >> > I don't want to.
> >> >
> >> > I asked "What can't be simulated?" because I assumed everything
> >> > necessary/mandatory could be simulated without needinng access to any
> >> > real EFI boot services.
> >> >
> >> > As far as I can see all that's necessary is to provide a compatible
> >> > interface.
> >>
> >> Could you be more precise what do you need? Please enumerate. UEFI spec has
> >> more than 2500 pages and I do not think that we need all stuff in dom0.
> >>
> >> > > What do you need from EFI boot services in dom0?
> >> >
> >> > The ability to call ExitBootServices() and SetVirtualAddressMap() on a
> >> > _virtual_ address map for _virtual_ services provided by the hypervisor.
> >>
> >> I am confused. Why do you need that? Please remember, EFI is owned and
> >> operated by Xen hypervisor. dom0 does not have direct access to EFI.
> >
> > Let's take a step back.
> >
> > My objection here is to passing the Dom0 kernel properties as if it were
> > booted with direct access to a full UEFI, then later fixing that up
> > (when Xen is detected and we apply its hypercall EFI implementation).
> >
>
> To be honest, I don't think that has ever been suggested here. What
> was suggested is to provide a minimal EFI like environment that allows
> the post-stub EFI code to proceed and find the ACPI root pointer.
>
> > If the kernel cannot use EFI natively, why pretend to the kernel that it
> > can? The hypercall implementation is _not_ EFI (though it provides
> > access to some services).
> >
>
> To get access to the ACPI root pointer, for which there is only one
> specified way of obtaining it on ARM, which is via the UEFI
> configuration table database
>
> > The two ways I can see providing Dom0 with EFI services are:
> >
> > * Have Xen create shims for any services, in which any hypercalls live,
> >   and pass these to the kernel with a virtual system table. This keeps
> >   the interface to the kernel the same regardless of Xen.
> >
> > * Have the kernel detect Xen EFI capability via Xen, without passing the
> >   usual native EFI parameters. This can then be installed into the
> >   kernel in a Xen-specific manner, and we know from the outset that
> >   Xen-specific caveats apply.
> >
> > As per my original email, I'm not against the renaming of the stub
> > parameters if we standardise the rest of the details, but I believe
> > that's orthogonal to the Xen Dom0 case.
> >
>
> Xen will not boot the kernel via the stub, but directly. It needs to
> supply a EFI like environment so that the kernel can boot via ACPI.
> There is no reason whatsoever to mock up boot services or other pieces
> of UEFI functionality that are not needed. The core kernel does not
> call any boot services or SetVirtualAddressMap/ConvertPointer, and
> there is already paravirtualized plumbing in place for the remaining
> runtime services.
>
> Hence my claim earlier that we should cope with the runtime services
> pointer being NULL, since that is really the only thing standing in

I suppose that you thought about EFI_INVALID_TABLE_ADDR...

> the way from the kernel side. If you feel that violates the spec in

If yes then you should know that dom0 on x86 EFI platform works
with efi.runtime == EFI_INVALID_TABLE_ADDR without any issue.
So, I think that all problems are solved.

Daniel

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