Re: [Xen-devel] Xen, EFI and kexec

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Nov 09, 2016 at 03:13:16PM +0100, Sylvain Munaut wrote:
> Hi,
> 
> 
> A bit of background: What I'm currently trying to do is to network
> boot a first linux image in EFI mode (using UEFI network boot), then
> from that image, kexec into Xen in EFI mode as well.
> 
> It's not working.
> 
> When you kexec into another linux in EFI mode, kexec actually passes
> some more infos to the kernel to allow it to use EFI mode properly.
> IIUC Linux also maps the various EFI ranges at deterministic addresses
> because the seting up EFI virtual address mode is only allowed once
> per boot and so both the old and new kernel have to use the same
> mappings.
> 
> I have seen the patch series by Daniel Kiper to add support for
> multiboot2 protocol and EFI support for it. This seems like a good
> step toward what I'd like to do. Adding multiboot2 support to kexec
> user tools seems like something I could do. This should allow Xen to
> get the info it needs.
> 
> However I don't think that'll be sufficient.
> 
> By the time kexec will launch Xen, the first kernel will have done an
> "ExitBootServices". And looking at Daniel's patch, that seems to be an

CC-ing Daniel.
> issue.

> 
> I'm also not sure if the EFI virtual address mode will be an issue or
> not yet. (i.e. Xen uses the physical address mode, but is that usable
> after the first linux setup the virtual mode).

OK, then that ought to work.
> 
> Is there anything else I overlooked that will be an issue ?

Buggy BIOSes?
> 
> Is this a use case worth supporting ? (Obviously I think so, but I'm biased 
> :p)

I think the idea of kexecing Xen is quite neat. You could (long-term with
lots of careful work), restart Xen without bringing down the guests.

(Yeah, I know, it sounds insane, but then I am livepatching an hypervisor
so I already have a foot in the looney bin)
> 
> And if I'm looking at it the wrong way, what way should I be looking at it ?
> 
> 
> Cheers,
> 
>     Sylvain Munaut
> 
> 
> PS: I'm currently making my way through the 2000+ pages of the EFI
> specs so I probably got some things wrong above ...
> 
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