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

On 12/27/2024 10:31 AM, Roman Kisel wrote:
> 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

Thank you for the persistence!

For the series,

Reviewed-by: Easwar Hariharan <eahariha@linux.microsoft.com>

On 12/27/2024 10:42 AM, Easwar Hariharan wrote:
> On 12/27/2024 10:31 AM, Roman Kisel wrote:

[...]

> 
> Thank you for the persistence!
I feel most fortunate learning from you, Michael, and Nuno :)
Thank you!

> 
> For the series,
> 
> Reviewed-by: Easwar Hariharan <eahariha@linux.microsoft.com>

-- 
Thank you,
Roman