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Re: [RFC PATCH 0/4] Physical address hypercall ABI ("HVMv2")
- To: Teddy Astie <teddy.astie@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 22 Apr 2025 09:45:10 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Tue, 22 Apr 2025 07:45:33 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 18.04.2025 16:18, Teddy Astie wrote:
> In current HVM mode, when a hypercall references a structure in guest memory,
> it is passed to the hypervisor as its "linear address" (e.g virtual address
> for
> the x86 long mode).
> One of the caveats is that this linear address (GVA) is generally not directly
> usable by the Xen and needs to be translated from GVA to GPA then HPA. This
> implies a complex and potentially expensive lookup of the pagetables inside
> the
> guest. This can be significant, especially if the P2M cannot use efficiently
> superpages (or with e.g XSA-304).
>
> This proposal introduce a new mode where all addresses used for hypercalls are
> GPADDR instead of GVADDR, therefore, looking up the HPA related to this GPA
> only needs a P2M lookup and not looking through the inside-guest pagetables.
>
> This mode is opt-in and must be enabled explicitely by the toolstack.
Which I view as a severe downside (leaving aside the PVH Dom0 aspect): This way
a guest needs to be converted all in one go. While doable, it'll be increasingly
risky with the size of the guest kernel code base.
A prior proposal of mine was to add an indicator to hypercall numbers (e.g. to
set the top bit there), to indicate which of the two models a particular
hypercall invocation uses.
Aiui Andrew had yet different (albeit also never spelled out) plans.
Jan
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