[v2,0/7] KVM: arm64: Use visibility hook to treat ID regs as RAZ

Message

Oliver Upton Sept. 2, 2022, 3:47 p.m. UTC

For reasons unknown, the Arm architecture defines the 64-bit views of
the 32-bit ID registers as UNKNOWN [1]. This combines poorly with the
fact that KVM unconditionally exposes these registers to userspace,
which could throw a wrench in migration between 64-bit only systems.

This series reworks KVM's definition of these registers to RAZ/WI with
the goal of providing consistent register values across 64-bit machines.

Patches 1-3 clean up the ID register accessors, taking advantage of the
fact that the generic accessors know how to handle RAZ.

Patches 4-6 start switch the handling of potentially nonzero AArch32 ID
registers to RAZ/WI. RAZ covers up the architecturally UNKNOWN values,
and WI allows for migration off of kernels that may provide garbage.
Note that hidden AArch32 ID registers continue to have RAZ behavior with
the additional expectation of invariance.

Lastly, patch 7 includes a small test for the issue.

Applies to 6.0-rc3. Tested with KVM selftests under the fast model w/
asymmetric 32 bit support and no 32 bit support whatsoever.

v1 -> v2:
 - Collect Reiji's r-b tags (thanks!)
 - Call sysreg_visible_as_raz() from read_id_reg() (Reiji)
 - Hoist sysreg_user_write_ignore() into kvm_sys_reg_set_user() (Reiji)

Oliver Upton (7):
  KVM: arm64: Use visibility hook to treat ID regs as RAZ
  KVM: arm64: Remove internal accessor helpers for id regs
  KVM: arm64: Drop raz parameter from read_id_reg()
  KVM: arm64: Spin off helper for calling visibility hook
  KVM: arm64: Add a visibility bit to ignore user writes
  KVM: arm64: Treat 32bit ID registers as RAZ/WI on 64bit-only system
  KVM: selftests: Add test for RAZ/WI AArch32 ID registers

 arch/arm64/kvm/sys_regs.c                     | 150 +++++++++---------
 arch/arm64/kvm/sys_regs.h                     |  24 ++-
 tools/testing/selftests/kvm/.gitignore        |   1 +
 tools/testing/selftests/kvm/Makefile          |   1 +
 .../kvm/aarch64/aarch64_only_id_regs.c        | 135 ++++++++++++++++
 5 files changed, 225 insertions(+), 86 deletions(-)
 create mode 100644 tools/testing/selftests/kvm/aarch64/aarch64_only_id_regs.c


base-commit: b90cb1053190353cc30f0fef0ef1f378ccc063c5

Comments

Oliver Upton Sept. 2, 2022, 3:52 p.m. UTC | #1

Lol, mess up my own copypasta:

On Fri, Sep 02, 2022 at 03:47:56PM +0000, Oliver Upton wrote:
> For reasons unknown, the Arm architecture defines the 64-bit views of
> the 32-bit ID registers as UNKNOWN [1]. This combines poorly with the
					^ on AArch64-only systems.

> fact that KVM unconditionally exposes these registers to userspace,
> which could throw a wrench in migration between 64-bit only systems.
> 
> This series reworks KVM's definition of these registers to RAZ/WI with
> the goal of providing consistent register values across 64-bit machines.
> 
> Patches 1-3 clean up the ID register accessors, taking advantage of the
> fact that the generic accessors know how to handle RAZ.
> 
> Patches 4-6 start switch the handling of potentially nonzero AArch32 ID
> registers to RAZ/WI. RAZ covers up the architecturally UNKNOWN values,
> and WI allows for migration off of kernels that may provide garbage.
> Note that hidden AArch32 ID registers continue to have RAZ behavior with
> the additional expectation of invariance.
> 
> Lastly, patch 7 includes a small test for the issue.
> 
> Applies to 6.0-rc3. Tested with KVM selftests under the fast model w/
> asymmetric 32 bit support and no 32 bit support whatsoever.

[1]: DDI0487H.a Table D12-2 'Instruction encodings for non-Debug System Register accesses'

v1: https://lore.kernel.org/kvmarm/[e-mail]/

--
Thanks,
Oliver

> v1 -> v2:
>  - Collect Reiji's r-b tags (thanks!)
>  - Call sysreg_visible_as_raz() from read_id_reg() (Reiji)
>  - Hoist sysreg_user_write_ignore() into kvm_sys_reg_set_user() (Reiji)
> 
> Oliver Upton (7):
>   KVM: arm64: Use visibility hook to treat ID regs as RAZ
>   KVM: arm64: Remove internal accessor helpers for id regs
>   KVM: arm64: Drop raz parameter from read_id_reg()
>   KVM: arm64: Spin off helper for calling visibility hook
>   KVM: arm64: Add a visibility bit to ignore user writes
>   KVM: arm64: Treat 32bit ID registers as RAZ/WI on 64bit-only system
>   KVM: selftests: Add test for RAZ/WI AArch32 ID registers
> 
>  arch/arm64/kvm/sys_regs.c                     | 150 +++++++++---------
>  arch/arm64/kvm/sys_regs.h                     |  24 ++-
>  tools/testing/selftests/kvm/.gitignore        |   1 +
>  tools/testing/selftests/kvm/Makefile          |   1 +
>  .../kvm/aarch64/aarch64_only_id_regs.c        | 135 ++++++++++++++++
>  5 files changed, 225 insertions(+), 86 deletions(-)
>  create mode 100644 tools/testing/selftests/kvm/aarch64/aarch64_only_id_regs.c
> 
> 
> base-commit: b90cb1053190353cc30f0fef0ef1f378ccc063c5
> -- 
> 2.37.2.789.g6183377224-goog
>