Re: ACPI NVS range conflicting with Dom0 page tables (or kernel image)

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Mon, Jun 17, 2024 at 01:22:37PM +0200, Jan Beulich wrote:
> Hello,
> 
> while it feels like we had a similar situation before, I can't seem to be
> able to find traces thereof, or associated (Linux) commits.

Is it some AMD Threadripper system by a chance? Previous thread on this
issue:
https://lore.kernel.org/xen-devel/CAOCpoWdOH=xGxiQSC1c5Ueb1THxAjH4WiZbCZq-QT+d_KAk3SA@xxxxxxxxxxxxxx/

> With
> 
> (XEN)  Dom0 kernel: 64-bit, PAE, lsb, paddr 0x1000000 -> 0x4000000
> ...
> (XEN)  Dom0 alloc.:   0000000440000000->0000000448000000 (619175 pages to be 
> allocated)
> ...
> (XEN)  Loaded kernel: ffffffff81000000->ffffffff84000000
> 
> the kernel occupies the space from 16Mb to 64Mb in the initial allocation.
> Page tables come (almost) directly above:
> 
> (XEN)  Page tables:   ffffffff84001000->ffffffff84026000
> 
> I.e. they're just above the 64Mb boundary. Yet sadly in the host E820 map
> there is
> 
> (XEN)  [0000000004000000, 0000000004009fff] (ACPI NVS)
> 
> i.e. a non-RAM range starting at 64Mb. The kernel (currently) won't tolerate
> such an overlap (also if it was overlapping the kernel image, e.g. if on the
> machine in question s sufficiently much larger kernel was used). Yet with its
> fundamental goal of making its E820 match the host one I'm also in trouble
> thinking of possible solutions / workarounds. I certainly do not see Xen
> trying to cover for this, as the E820 map re-arrangement is purely a kernel
> side decision (forward ported kernels got away without, and what e.g. the
> BSDs do is entirely unknown to me).

In Qubes we have worked around the issue by moving the kernel lower
(CONFIG_PHYSICAL_START=0x200000):
https://github.com/QubesOS/qubes-linux-kernel/commit/3e8be4ac1682370977d4d0dc1d782c428d860282

Far from ideal, but gets it bootable...

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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