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
