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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 14 Sep 2015, Mark Rutland 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).
> 
> 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).
> 
> 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.

A not a fan of three-point estimates, so I am just going to say that
"this looks like a lot of work". Also emulating services is known to be
prone to errors.


> * 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.

I prefer this approach by far. In the future we might have to move the
xen_early_init call earlier (before ACPI and EFI Runtime Services get
initialized).


> 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.

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