Re: [PATCH v2 3/8] KVM: arm64: Remove VHE host restore of CPACR_EL1.ZEN

On Thu, Feb 06, 2025 at 02:10:57PM +0000, Mark Rutland wrote:

When KVM is in VHE mode, the host kernel tries to save and restore the
configuration of CPACR_EL1.ZEN (i.e. CPTR_EL2.ZEN when HCR_EL2.E2H=1)
across kvm_arch_vcpu_load_fp() and kvm_arch_vcpu_put_fp(), since the
configuration may be clobbered by hyp when running a vCPU. This logic is
currently redundant.

The VHE hyp code unconditionally configures CPTR_EL2.ZEN to 0b01 when
returning to the host, permitting host kernel usage of SVE.

Now that the host eagerly saves and unbinds its own FPSIMD/SVE/SME
state, there's no need to save/restore the state of the EL0 SVE trap.
The kernel can safely save/restore state without trapping, as described
above, and will restore userspace state (including trap controls) before
returning to userspace.

Remove the redundant logic.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Fuad Tabba <redacted>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Oliver Upton <redacted>
Cc: Will Deacon <will@kernel.org>
---
 arch/arm64/include/asm/kvm_host.h |  1 -
 arch/arm64/kvm/fpsimd.c           | 16 ----------------
 2 files changed, 17 deletions(-)

Acked-by: Will Deacon <will@kernel.org>

Will