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    SubjectRe: [PATCH] efi/libstub/fdt: Standardize the names of EFI stub parameters
    On Thu, 10 Sep 2015, Mark Rutland wrote:
    > On Thu, Sep 10, 2015 at 02:52:25PM +0100, Stefano Stabellini wrote:
    > > On Thu, 10 Sep 2015, Mark Rutland wrote:
    > > > On Thu, Sep 10, 2015 at 12:37:57PM +0100, Stefano Stabellini wrote:
    > > > > 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).
    > > >
    > > > Sure, but it feels odd to provide the usual information in this manner
    > > > if it cannot be used. If you require Xen-specific code to make things
    > > > work, I would imagine this information could be dciscovered in a
    > > > Xen-specific manner.
    > >
    > > We need ACPI (or Device Tree) to find that Xen is available on the
    > > platform, so we cannot use Xen-specific code to get the ACPI tables.
    >
    > I don't understand. The proposition already involves passing a custom DT
    > to the OS, implying that Xen knows how to boot that OS, and how to pass
    > it a DTB.
    >
    > Consider:
    >
    > A) In the DT-only case, we go:
    >
    > DT -> Discover Xen -> Xen-specific stuff
    >
    >
    > B) The proposition is that un the ACPI case we go:
    >
    > DT -> EFI tables -> ACPI tables -> Discover Xen -> Xen-specific stuff -> override EFI/ACPI stuff
    > \-----------------------/
    > (be really cautious here)

    Well, yes. To be pedantic "override" here would just be the different
    delivery method for RuntimeServices. I guess it still counts.


    > 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()?

    If not, we could just go for this. A lot of complexity would go away.


    > 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.


    >
    > E) Partially-generic option:
    > EFI -> EFI application -> Xen detected by registered GUID -> Xen-specific EFI bootloader stuff -> OS in Xen-specific configuration
    >
    >
    > > > > In any case this should be separate from the shim ABI discussion.
    > > >
    > > > I disagree; I think this is very much relevant to the ABI discussion.
    > > > That's not to say that I insist on a particular approach, but I think
    > > > that they need to be considered together.
    > >
    > > Let's suppose Xen didn't expose any RuntimeServices at all, would that
    > > make it easier to discuss about the EFI stub parameters?
    >
    > It would simply the protocol specific to Xen, certainly.
    >
    > > In the grant scheme of things, they are not that important, as Ian
    > > wrote what is important is how to pass the RSDP.
    >
    > Unfortunately we're still going to have to care about this eventually,
    > even if for something like kexec. So we still need to spec out the state
    > of things if this is going to be truly generic.

    Fair enough. My position is that if we restrict this to RuntimeServices,
    it might be possible, but I still prefer C).


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