Re: [Xen-devel] REGRESSION: Xen 4.13 RC5 fails to bootstrap Dom0 on ARM

[Roman Shaposhnik <roman@xxxxxxxxxx>]

On Wed, Dec 18, 2019 at 3:50 AM Julien Grall <julien@xxxxxxx> wrote:
>
> Hi,
>
> On 18/12/2019 07:36, Roman Shaposhnik wrote:
> > On Tue, Dec 17, 2019 at 6:56 PM Roman Shaposhnik <roman@xxxxxxxxxx> wrote:
> >> Exactly! That's the other surprising bit -- I noticed that too -- its not 
> >> like
> >> Xen doesn't see any of the memory above 1G -- it just doesn't see enough 
> >> of it.
> >>
> >> So the question is -- what is Linux doing that Xen doesn't?
> >
> > By the way, speaking of running Xen under ARM/qemu -- here's an interesting
> > observation: when I run qemu-system-aarch64 with -m 4096 option it seems
> > that, again, Linux kernel is perfectly content with having access to 4G of 
> > RAM,
> > while Xen only sees about 2G.
>
> Linux and Xen should see close to the same amount as memory as long as
> you are using the same bootloader...

Thanks for confirming. This is what I'm trying to get to on this
thread. Any help
would be greatly appreciated!

> > This may actually have something to do with UEFI I guess.
>
> ...  could you confirm whether you are booting Linux using UEFI or not?

The boot sequence in both cases is:
   HiKey l-loader
   HiKey Tianocore EDK2 – UEFI
   GRUB (as a UEFI payload)
   Xen | Linux

GRUB's commands for booting Xen + Dom0:
    xen_hypervisor /boot/xen.efi console=dtuart   dom0_mem=640M
dom0_max_vcpus=1 dom0_vcpus_pin
    xen_module /boot/kernel console=hvc0 root=(hd1,gpt1)/rootfs.img text
    devicetree (hd1,gpt4)/eve.dtb
    xen_module (hd1,gpt1)/initrd.img

GRUB's commands for booting Linux only:
    linux /boot/kernel  console=ttyAMA0 console=ttyAMA1
console=ttyAMA2 console=ttyAMA3
root=PARTUUID=f71bd987-d99a-4c88-9781-cf4c26cae55e rootdelay=3
    devicetree (hd1,gpt4)/eve.dtb

So -- nothing boots directly by UEFI -- everything goes through GRUB.

However, my understanding is that GRUB will detect devicetree
information provided by UEFI (even though devicetree command is
supposed to completely replace that). Hence it is possible that Linux
relies on some residuals left in memory by GRUB that Xen doesn't pay
attention to (but this is a pretty wild speculation only).

Thanks,
Roman.

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