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). -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
