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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 10 Sep 2015, Mark Rutland wrote:
> > > Does Xen not talk to EFI itself and/or give the kernel a virtual EFI
> > > interface?
> > 
> > Xen talks to EFI itself but the interface provided to dom0 is somewhat
> > different: there are no BootServices (Xen calls ExitBootServices before
> > running the kernel), and the RuntimeServices go via hypercalls (see
> > drivers/xen/efi.c).
> 
> That's somewhat hideous; a non Xen-aware OS wouild presumably die if
> trying to use any runtime services the normal way? I'm not keen on
> describing things that the OS cannot use.
 
I agree that is somewhat hideous, but a non-Xen aware OS traditionally
has never been able to even boot as Dom0. On ARM it can, but it still
wouldn't be very useful (one couldn't use it to start other guests).


> Why can't Xen give a virtual EFI interface to Dom0 / guests? e.g.
> create pages of RuntimeServicesCode that are trivial assembly shims
> doing hypercalls, and plumb these into the virtual EFI memory map and
> tables?
> 
> That would keep things sane for any guest, allow for easy addition of
> EFI features, and you could even enter the usual EFI entry point,
> simulate ExitBootServices(), SetVirtualAddressMap(), and allow the guest
> to make things sane for itself...

That's the way it was done on x86 and now we have common code both in
Linux (drivers/xen/efi.c) and Xen (xen/common/efi) which implement this
scheme.  Switching to a different solution for ARM, would mean diverging
with x86, which is not nice, or reimplementing the x86 solution too,
which is expensive.

BTW I think that the idea you proposed was actually considered at the
time and deemed hard to implement, if I recall correctly.


In any case this should be separate from the shim ABI discussion.

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