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Re: [PATCH] x86: invoke xenheap_max_mfn() unconditionally
- To: George Dunlap <gwd@xxxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 29 Oct 2026 12:17:00 +0100
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, Roger Pau Monné <roger@xxxxxxxxxxxxxx>
- Delivery-date: Thu, 29 Oct 2026 11:17:14 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 29.10.2026 11:56, George Dunlap wrote:
> On Mon, Oct 12, 2026 at 12:12 PM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>
>> Calling the function only conditionally can, when highmem_start isn't set
>> and when hotplugged memory appears above the boundary where the "short"
>> directmap ends, result in Xenheap allocations to yield memory which isn't
>> accessible through the "short" directmap. Accesses from PV guest context
>> would then either fault or access guest controlled memory.
>>
>> With this the bifurcation done by 88e3ed61642b ("x86/NUMA: make
>> init_node_heap() respect Xen heap limit") can also be undone:
>> end_boot_allocator() can be called from a single place, and system_state
>> is bumped right afterwards.
>>
>> Fixes: adc5afbf1c70 ("x86: support up to 16Tb")
>> Assisted-by: Claude Opus 5.5
>> Reported-by: George Dunlap <gwd@xxxxxxxxxxxxxx>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> Reviewed-by: George Dunlap <gwd@xxxxxxxxxxxxxx>
Thanks.
> As an aside, Claude notes that highmem_start isn't clamped to 5TiB; in
> theory if somebody put `highmem-start=6T` on the command-line, it
> would cause problems.
True, and we may want to do better there. Still various other cmdline
options, when used with inappropriate values, will cause mischief as
well.
Jan
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