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Re: [PATCH v2] x86/domain: adjust limitation on shared_info allocation below 4G
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 4 Feb 2026 16:08:21 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Wed, 04 Feb 2026 15:08:44 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 04.02.2026 15:52, Roger Pau Monné wrote:
> On Wed, Feb 04, 2026 at 03:06:52PM +0100, Jan Beulich wrote:
>> On 04.02.2026 13:25, Roger Pau Monne wrote:
>>> The limitation of shared_info being allocated below 4G to fit in the
>>> start_info field only applies to 32bit PV guests. On 64bit PV guests the
>>> start_info field is 64bits wide. HVM guests don't use start_info at all.
>>>
>>> Drop the restriction in arch_domain_create() and instead free and
>>> re-allocate the page from memory below 4G if needed in switch_compat(),
>>> when the guest is set to run in 32bit PV mode.
>>>
>>> Fixes: 3cadc0469d5c ("x86_64: shared_info must be allocated below 4GB as it
>>> is advertised to 32-bit guests via a 32-bit machine address field in
>>> start_info.")
>>
>> The tag is here because there is the (largely theoretical?) possibility for
>> a system to have no memory at all left below 4Gb, in which case creation of
>> a PV64 or non-shadow HVM guest would needlessly fail?
>
> It's kid of an issue we discovered when using strict domain NUMA node
> placement. At that point the toolstack would exhaust all memory on
> node 0 and by doing that inadvertently consume all memory below 4G.
Right, and hence also my "memory: arrange to conserve on DMA reservation",
where I'm still fighting with myself as to what to do with the comments you
gave there.
Jan
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