Allow userspace to downgrade EL2 features (TWED, HCX) to ensure VM live
migration compatibility between hosts with varying feature support.

Although ID_AA64MMFR1_EL1.VH is an EL2 feature, it is still kept as non-
writable in userspace. When FEAT_E2H0 is not implemented, HCR_EL2.E2H is
res1. When E2H=1, EL2 operates in VHE mode. In this case, if userspace
modifies the VH bit to 0, the Guest's kernel will still run in vEL2, leading
to inconsistency. 

v1: https://lore.kernel.org/linux-arm-kernel/20250325121126.1380681-1-yangjinqian1@huawei.com/
v2: https://lore.kernel.org/linux-arm-kernel/20250909034415.3822478-1-yangjinqian1@huawei.com/

Change Log:
v1->v2:
  -Added support for downgrading TWED and VH fields.
  -Added corresponding test cases to the set_id_regs selftest.

v2->v3:
  -Kept VH non-writable.
  -Separated kernel patches and self-test patches.

Jinqian Yang (2):
  KVM: arm64: Make ID_AA64MMFR1_EL1.{HCX, TWED} writable from userspace
  KVM: arm64: selftests: Test writes to ID_AA64MMFR1_EL1.{HCX, TWED}

 arch/arm64/kvm/sys_regs.c                       | 2 --
 tools/testing/selftests/kvm/arm64/set_id_regs.c | 2 ++
 2 files changed, 2 insertions(+), 2 deletions(-)

-- 
2.33.0

On Thu, 11 Sep 2025 19:46:19 +0800, Jinqian Yang wrote:
> Allow userspace to downgrade EL2 features (TWED, HCX) to ensure VM live
> migration compatibility between hosts with varying feature support.
> 
> Although ID_AA64MMFR1_EL1.VH is an EL2 feature, it is still kept as non-
> writable in userspace. When FEAT_E2H0 is not implemented, HCR_EL2.E2H is
> res1. When E2H=1, EL2 operates in VHE mode. In this case, if userspace
> modifies the VH bit to 0, the Guest's kernel will still run in vEL2, leading
> to inconsistency.
> 
> [...]

Applied to next, thanks!

[1/2] KVM: arm64: Make ID_AA64MMFR1_EL1.{HCX, TWED} writable from userspace
      commit: 1a0b2bf6ff11d2e0438f3a7d1d299339edab96e8
[2/2] KVM: arm64: selftests: Test writes to ID_AA64MMFR1_EL1.{HCX, TWED}
      commit: be8c9192eaeee21fdaacd72ad4ba992b9e42377b

Cheers,

	M.
-- 
Without deviation from the norm, progress is not possible.