Re: [Xen-devel] [PATCH v3 1/2] x86/setup: do not relocate Xen over current Xen image placement

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.01.18 at 14:05, <daniel.kiper@xxxxxxxxxx> wrote:
> Otherwise, due to Xen code/data changes under CPU feet, Xen may crash
> silently at boot.
> 
> We were hit by the issue in OVS Xen 4.4 with my earlier version of
> EFI/Multiboot2 patches. Initially its implementation allowed relocation
> of Xen even if it was relocated by the bootloader. This led to the
> crashes on some new Oracle machines because copy destination partially
> overlapped with the end of current/initial Xen image placement.
> 
> After some discussion on Xen-devel we decided to disable Xen relocation in
> my EFI/Multiboot2 upstream patches if the booloader did the work for us.
> Though one case is still not covered. If Xen is not relocated by the
> booloader then it tries to do that by itself. If all RAM regions above
> currently occupied one are unsuitable for relocation then Xen tries to move
> itself higher in it. And if (end - reloc_size + XEN_IMG_OFFSET) goes below
> __pa(_end) then copy/relocation destination overlaps, at least partially,
> with its source.
> 
> I can agree that this should not happen on todays machines very often.
> If at all. It is rather unusual to not have usable RAM regions above
> ~5 MiB nowadays. Though I think that we should at least consider putting
> such safety measure here. Otherwise Xen may crash mysteriously without
> any stack trace. It is very confusing and impairs further debugging.
> 
> Signed-off-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>

Acked-by: Jan Beulich <jbeulich@xxxxxxxx>



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