[PATCH v4 0/5] hyperv: Fixes for get_vtl(), hv_vtl_apicid_to_vp_id()

[Roman Kisel <romank@linux.microsoft.com>]

The get_vtl(void) function

* has got one bug when the code started using a wrong pointer type after
  refactoring, and also
* the both function in question don't adhere to the requirements of
  the Hypervisor Top-Level Funactional Specification[1, 2] as the code overlaps
  the input and output areas for a hypercall.

The first issue leads to a wrong 100% reproducible computation due to reading
a byte worth of data at a wrong offset. That in turn leads to using a nonsensical
value ("fortunately", could catch it easily!) for the current VTL when initiating
VMBus communications. As a repercussion from that, the system wouldn't boot. The
fix is straightforward: use the correct pointer type.

The second issue doesn't seem to lead to any reproducible breakage just yet. It is
fixed with using the output hypercall pages allocated per-CPU, and that isn't the
only or the most obvious choice so let me elaborate why that fix appears to be the
best one in my opinion out of the options I could conceive of.

The approach chosen for fixing the second issue makes two things shine through:

* these functions just get a vCPU register, no special treatment needs to be
  involved,
* VTLs and dom0 can and should share code as both exist to provide services to
  a guest(s), be that from within the partition or from outside of it.

The projected benefits include replacing the functions in question with a future
`hv_get_vp_registers` one shared between dom0 and VTLs to allow for a better test
coverage.

I have validated the fixes by booting the fixed kernel in VTL2 up using OpenVMM and
OpenHCL[3, 4].

[1] https://learn.microsoft.com/en-us/virtualization/hyper-v-on-windows/tlfs/hypercall-interface
[2] https://github.com/MicrosoftDocs/Virtualization-Documentation/tree/main/tlfs
[3] https://openvmm.dev/guide/user_guide/openhcl.html
[4] https://github.com/microsoft/OpenVMM

[v4]
  - Wrapped DECLARE_FLEX_ARRAY into a struct and added one more
    member as the documentation requires,
  - Removed superfluous type coercion,
  - Fixed tags,
  - Rebased onto the latest hyperv-next branch.

[v3]: https://lore.kernel.org/lkml/20241226213110.899497-1-romank@linux.microsoft.com/
  - Added a fix for hv_vtl_apicid_to_vp_id(),
  - Split out the patch for enabling the hypercall output page,
  - Updated the title of the patch series,

[v2]: https://lore.kernel.org/lkml/20241226203050.800524-1-romank@linux.microsoft.com/
  - Used the suggestions to define an additional structure to improve code readability,
  - Split out the patch with that definition.

[v1]: https://lore.kernel.org/lkml/20241218205421.319969-1-romank@linux.microsoft.com/

Roman Kisel (5):
  hyperv: Define struct hv_output_get_vp_registers
  hyperv: Fix pointer type in get_vtl(void)
  hyperv: Enable the hypercall output page for the VTL mode
  hyperv: Do not overlap the hvcall IO areas in get_vtl()
  hyperv: Do not overlap the hvcall IO areas in hv_vtl_apicid_to_vp_id()

 arch/x86/hyperv/hv_init.c   |  6 ++--
 arch/x86/hyperv/hv_vtl.c    |  2 +-
 drivers/hv/hv_common.c      |  6 ++--
 include/hyperv/hvgdk_mini.h | 65 +++++++++++++++++++++++++++++++++++--
 4 files changed, 70 insertions(+), 9 deletions(-)


base-commit: 26e1b813fcd02984b1cac5f3decdf4b0bb56fe02
-- 
2.34.1