[PATCH 3/4] arm64: KVM: Install stage-2 translation before enabling traps on VHE

[christoffer.dall@arm.com (Christoffer Dall)]

On Mon, Nov 05, 2018 at 02:36:15PM +0000, Marc Zyngier wrote:
> It is a bit odd that we only install stage-2 translation after having
> cleared HCR_EL2.TGE, which means that there is a window during which
> AT requests could fail as stage-2 is not configured yet.
> 
> Let's move stage-2 configuration before we clear TGE, making the
> guest entry sequence clearer: we first configure all the guest stuff,
> then only switch to the guest translation regime.
> 
> Non-VHE doesn't have that kind of behaviour, and is left alone.

I'm a bit confused about this statement.  You can still issue a S12E1x
AT instruction after activating traps (setting HCR_EL2.VM) on non-VHE
and get at the same behavior, right?

Is the point here that we are not aware of any non-VHE implementations
that speculate AT instructions in this window, or am I missing some
architectural nugget that prevents problems on non-VHE systems?

In any case, why not change the non-VHE code as well to preserve
symmetry for both types of systems?


Thanks,

    Christoffer

> 
> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> ---
>  arch/arm64/kvm/hyp/switch.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> index 7cc175c88a37..51d5d966d9e5 100644
> --- a/arch/arm64/kvm/hyp/switch.c
> +++ b/arch/arm64/kvm/hyp/switch.c
> @@ -499,8 +499,8 @@ int kvm_vcpu_run_vhe(struct kvm_vcpu *vcpu)
>  
>  	sysreg_save_host_state_vhe(host_ctxt);
>  
> -	__activate_traps(vcpu);
>  	__activate_vm(vcpu->kvm);
> +	__activate_traps(vcpu);
>  
>  	sysreg_restore_guest_state_vhe(guest_ctxt);
>  	__debug_switch_to_guest(vcpu);
> -- 
> 2.19.1
>