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Re: [PATCH v3] x86/hyperv: use dynamically allocated page for hypercalls
- To: Jan Beulich <jbeulich@xxxxxxxx>, Ariadne Conill <ariadne@ariadne.space>
- From: Alejandro Vallejo <agarciav@xxxxxxx>
- Date: Tue, 29 Apr 2025 11:48:14 +0100
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- Cc: Paul Durrant <paul@xxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, "Alexander M . Merritt" <alexander@xxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 29 Apr 2025 10:48:22 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Tue Apr 29, 2025 at 9:28 AM BST, Jan Beulich wrote:
> On 28.04.2025 21:57, Ariadne Conill wrote:
>> Previously Xen placed the hypercall page at the highest possible MFN,
>> but this caused problems on systems where there is more than 36 bits
>> of physical address space.
>
> Hmm, I should have asked already on the earlier version: What kinds of
> problems are these, beyond ...
>
>> In general, it also seems unreliable to assume that the highest possible
>> MFN is not already reserved for some other purpose.
>
> ... this particular aspect? I find it puzzling that such problems would
> depend on the number of physical address bits.
>
> Jan
Pagefault on access (due to reserved bits being set) on access to the
hypercall page. The available guest-physical address space doesn't seem
to be as wide as advertised, though I didn't carry enough tests to
single this as the only explanation. Seeing how we don't really know
what's already on the last mfn this seems like a strict improvement
irrespective of the actual cause of the fault.
Cheers,
Alejandro
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