[v3,0/7] KVM: arm64: Use visibility hook to treat ID regs as RAZ

Message

Oliver Upton Sept. 13, 2022, 9:44 a.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.

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

v2: https://lore.kernel.org/kvmarm/[e-mail]/

v2 -> v3:
 - Collect more of Reiji's r-bs (thanks again!)
 - Test the RAZ+invariant registers (AFR0, DFR1, unallocated AA32 ID
   registers) (Drew)
 - Give the selftest a more sensible name

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 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 +
 .../selftests/kvm/aarch64/aarch32_id_regs.c   | 169 ++++++++++++++++++
 5 files changed, 259 insertions(+), 86 deletions(-)
 create mode 100644 tools/testing/selftests/kvm/aarch64/aarch32_id_regs.c


base-commit: b90cb1053190353cc30f0fef0ef1f378ccc063c5

Comments

Marc Zyngier Sept. 14, 2022, 10:41 a.m. UTC | #1

On Tue, 13 Sep 2022 09:44:33 +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
> 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.
> 
> [...]

Applied to kvm-arm64/next, thanks!

[1/7] KVM: arm64: Use visibility hook to treat ID regs as RAZ
      commit: 34b4d20399e6fad2e3379b11e68dff1d1549274e
[2/7] KVM: arm64: Remove internal accessor helpers for id regs
      commit: 4782ccc8ef50fabb70bab9fa73186285dba6d91d
[3/7] KVM: arm64: Drop raz parameter from read_id_reg()
      commit: cdd5036d048ca96ef5212fb37f4f56db40cb1bc2
[4/7] KVM: arm64: Spin off helper for calling visibility hook
      commit: 5d9a718b64e428a40939806873ecf16f072008b3
[5/7] KVM: arm64: Add a visibility bit to ignore user writes
      commit: 4de06e4c1dc949c35c16e4423b4ccd735264b0a9
[6/7] KVM: arm64: Treat 32bit ID registers as RAZ/WI on 64bit-only system
      commit: d5efec7ed826b3b29c6847bf59383d8d07347a4e
[7/7] KVM: selftests: Add test for AArch32 ID registers
      commit: 797b84517c190053597e3f7e03ead15da872e04d

Cheers,

	M.