Re: [PATCH v2 2/8] KVM: arm64: Remove host FPSIMD saving for non-protected KVM

[Will Deacon <will@kernel.org>]

On Thu, Feb 06, 2025 at 02:10:56PM +0000, Mark Rutland wrote:
> Now that the host eagerly saves its own FPSIMD/SVE/SME state,
> non-protected KVM never needs to save the host FPSIMD/SVE/SME state,
> and the code to do this is never used. Protected KVM still needs to
> save/restore the host FPSIMD/SVE state to avoid leaking guest state to
> the host (and to avoid revealing to the host whether the guest used
> FPSIMD/SVE/SME), and that code needs to be retained.
> 
> Remove the unused code and data structures.
> 
> To avoid the need for a stub copy of kvm_hyp_save_fpsimd_host() in the
> VHE hyp code, the nVHE/hVHE version is moved into the shared switch
> header, where it is only invoked when KVM is in protected mode.
> 
> 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       | 20 +++++-------------
>  arch/arm64/kvm/arm.c                    |  8 -------
>  arch/arm64/kvm/fpsimd.c                 |  2 --
>  arch/arm64/kvm/hyp/include/hyp/switch.h | 25 ++++++++++++++++++++--
>  arch/arm64/kvm/hyp/nvhe/hyp-main.c      |  2 +-
>  arch/arm64/kvm/hyp/nvhe/switch.c        | 28 -------------------------
>  arch/arm64/kvm/hyp/vhe/switch.c         |  8 -------
>  7 files changed, 29 insertions(+), 64 deletions(-)

[...]

> diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
> index f838a45665f26..c5b8a11ac4f50 100644
> --- a/arch/arm64/kvm/hyp/include/hyp/switch.h
> +++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
> @@ -375,7 +375,28 @@ static inline void __hyp_sve_save_host(void)
>  			 true);
>  }
>  
> -static void kvm_hyp_save_fpsimd_host(struct kvm_vcpu *vcpu);
> +static void kvm_hyp_save_fpsimd_host(struct kvm_vcpu *vcpu)
> +{
> +	/*
> +	 * Non-protected kvm relies on the host restoring its sve state.
> +	 * Protected kvm restores the host's sve state as not to reveal that
> +	 * fpsimd was used by a guest nor leak upper sve bits.
> +	 */
> +	if (system_supports_sve()) {
> +		__hyp_sve_save_host();
> +
> +		/* Re-enable SVE traps if not supported for the guest vcpu. */
> +		if (!vcpu_has_sve(vcpu))
> +			cpacr_clear_set(CPACR_EL1_ZEN, 0);
> +
> +	} else {
> +		__fpsimd_save_state(host_data_ptr(host_ctxt.fp_regs));
> +	}
> +
> +	if (kvm_has_fpmr(kern_hyp_va(vcpu->kvm)))
> +		*host_data_ptr(fpmr) = read_sysreg_s(SYS_FPMR);
> +}
> +
>  
>  /*
>   * We trap the first access to the FP/SIMD to save the host context and
> @@ -425,7 +446,7 @@ static bool kvm_hyp_handle_fpsimd(struct kvm_vcpu *vcpu, u64 *exit_code)
>  	isb();
>  
>  	/* Write out the host state if it's in the registers */
> -	if (host_owns_fp_regs())
> +	if (is_protected_kvm_enabled() && host_owns_fp_regs())
>  		kvm_hyp_save_fpsimd_host(vcpu);

I wondered briefly whether this would allow us to clean up the CPACR
handling a little and avoid the conditional SVE trap re-enabling inside
kvm_hyp_save_fpsimd_host() but I couldn't come up with a clean way to
do it without an additional ISB. Hrm.

Anyway, as far as the patch goes:

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

Will