[RFC,0/4] Physical address hypercall ABI ("HVMv2")

Message

Teddy Astie April 18, 2025, 2:18 p.m. UTC

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.

This is also mandatory for confidential-computing guests (e.g SEV) where the
guest pagetable are not visible to the hypervisor.

In a synthetic xtf-based hypercall benchmark (VCPUOP_get_runstate_info loop),
it gives broadly a 30% overhead reduction when tested on a AMD EPYC 9124.

This serie only implement support for x86 yet this ABI is also meaningful for
other architectures as well.
A separate patch for adding support in Linux is planned.

Teddy Astie (4):
  xen: Introduce physaddr_abi CDF flag
  x86/hvm: Consider phyaddr_abi when copying from/to guest memory
  x86/public: Expose physaddr_abi through Xen HVM CPUID leaf
  libxl: Add support for enabling physaddr_abi

 tools/include/libxl.h               |  5 +++++
 tools/libs/light/libxl_create.c     |  4 ++++
 tools/libs/light/libxl_types.idl    |  1 +
 tools/xl/xl_parse.c                 |  1 +
 xen/arch/x86/cpuid.c                |  3 +++
 xen/arch/x86/hvm/hvm.c              | 17 ++++++++++++++---
 xen/common/domain.c                 | 10 +++++++++-
 xen/include/public/arch-x86/cpuid.h |  2 ++
 xen/include/public/domctl.h         |  4 +++-
 xen/include/xen/sched.h             |  6 ++++++
 10 files changed, 48 insertions(+), 5 deletions(-)