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Re: [PATCH] x86/hyperv: Adjust hypercall page placement
- To: Alejandro Vallejo <agarciav@xxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Alejandro Vallejo <agarciav@xxxxxxx>
- Date: Thu, 24 Apr 2025 14:47:46 +0100
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- Cc: Jason Andryuk <jason.andryuk@xxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Paul Durrant <paul@xxxxxxx>
- Delivery-date: Thu, 24 Apr 2025 13:48:14 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Thu Apr 24, 2025 at 1:45 PM BST, Alejandro Vallejo wrote:
> Xen nowadays crashes under some Hyper-V configurations when
> paddr_bits>36. At the 44bit boundary we reach an edge case in which the
> end of the guest physical address space is not representable using 32bit
> MFNs. Furthermore, it's an act of faith that the tail of the physical
> address space has no reserved regions already.
>
> This commit uses the first unused MFN rather than the last, thus
> ensuring the hypercall page placement is more resilient against such
> corner cases.
>
> While at this, add an extra BUG_ON() to explicitly test for the
> hypercall page being correctly set, and mark hcall_page_ready as
> __ro_after_init.
>
> Fixes: 620fc734f854("x86/hyperv: setup hypercall page")
> Signed-off-by: Alejandro Vallejo <agarciav@xxxxxxx>
After a side discussion, this seems on the unsafe side of things due to
potential collision with MMIO. I'll resend (though not today) with the
page overlapping a RAM page instead. Possibly the last page of actual
RAM.
Cheers,
Alejandro
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