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

[Will Deacon <will@kernel.org>]

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 <tabba@google.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Mark Brown <broonie@kernel.org>
> Cc: Oliver Upton <oliver.upton@linux.dev>
> 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