Re: [PATCH 03/11] KVM: arm64: nv: Load guest FP state for ZCR_EL2 trap

[Marc Zyngier <maz@kernel.org>]

On Sat, 01 Jun 2024 00:13:50 +0100,
Oliver Upton <oliver.upton@linux.dev> wrote:
> 
> Round out the ZCR_EL2 gymnastics by loading SVE state in the fast path
> when the guest hypervisor tries to access SVE state.
> 
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> ---
>  arch/arm64/kvm/hyp/include/hyp/switch.h | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
> index 428ee15dd6ae..5872eaafc7f0 100644
> --- a/arch/arm64/kvm/hyp/include/hyp/switch.h
> +++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
> @@ -345,6 +345,10 @@ static bool kvm_hyp_handle_fpsimd(struct kvm_vcpu *vcpu, u64 *exit_code)
>  		if (guest_hyp_fpsimd_traps_enabled(vcpu))
>  			return false;
>  		break;
> +	case ESR_ELx_EC_SYS64:
> +		if (WARN_ON_ONCE(!is_hyp_ctxt(vcpu)))
> +			return false;
> +		fallthrough;
>  	case ESR_ELx_EC_SVE:
>  		if (!sve_guest)
>  			return false;
> @@ -520,6 +524,22 @@ static bool handle_ampere1_tcr(struct kvm_vcpu *vcpu)
>  	return true;
>  }
>  
> +static bool kvm_hyp_handle_zcr(struct kvm_vcpu *vcpu, u64 *exit_code)
> +{
> +	u32 sysreg = esr_sys64_to_sysreg(kvm_vcpu_get_esr(vcpu));
> +
> +	if (!vcpu_has_nv(vcpu))
> +		return false;
> +
> +	if (sysreg != SYS_ZCR_EL2)
> +		return false;
> +
> +	if (guest_owns_fp_regs())
> +		return false;
> +
> +	return kvm_hyp_handle_fpsimd(vcpu, exit_code);

For my own understanding of the flow: let's say the L1 guest accesses
ZCR_EL2 while the host own the FP regs:

- ZCR_EL2 traps
- we restore the guest's state, enable SVE
- ZCR_EL2 traps again
- emulate the access on the slow path

In contrast, the same thing using ZCR_EL1 in L1 results in:

- ZCR_EL1 traps
- we restore the guest's state, enable SVE

and we're done.

Is that correct? If so, a comment would help... ;-)

> +}
> +
>  static bool kvm_hyp_handle_sysreg(struct kvm_vcpu *vcpu, u64 *exit_code)
>  {
>  	if (cpus_have_final_cap(ARM64_WORKAROUND_CAVIUM_TX2_219_TVM) &&
> @@ -537,6 +557,9 @@ static bool kvm_hyp_handle_sysreg(struct kvm_vcpu *vcpu, u64 *exit_code)
>  	if (kvm_hyp_handle_cntpct(vcpu))
>  		return true;
>  
> +	if (kvm_hyp_handle_zcr(vcpu, exit_code))
> +		return true;
> +
>  	return false;
>  }
>  

Thanks,

	M.

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