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


  • To: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Mon, 17 Jun 2024 13:22:37 +0200
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  • Cc: Juergen Gross <jgross@xxxxxxxx>
  • Delivery-date: Mon, 17 Jun 2024 11:22:45 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

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.

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).

Jan



 


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