[PATCH v3 09/20] KVM: arm/arm64: mask/unmask daif around VHE guests

[Marc Zyngier]

Hi James,

On Thu, Oct 05 2017 at  8:18:01 pm BST, James Morse <james.morse at arm.com> wrote:
> Non-VHE systems take an exception to EL2 in order to world-switch into the
> guest. When returning from the guest KVM implicitly restores the DAIF
> flags when it returns to the kernel at EL1.
>
> With VHE none of this exception-level jumping happens, so KVMs
> world-switch code is exposed to the host kernel's DAIF values, and KVM
> spills the guest-exit DAIF values back into the host kernel.
> On entry to a guest we have Debug and SError exceptions unmasked, KVM
> has switched VBAR but isn't prepared to handle these. On guest exit
> Debug exceptions are left disabled once we return to the host and will
> stay this way until we enter user space.
>
> Add a helper to mask/unmask DAIF around VHE guests. The unmask can only
> happen after the hosts VBAR value has been synchronised by the isb in
> __vhe_hyp_call (via kvm_call_hyp()). Masking could be as late as
> setting KVMs VBAR value, but is kept here for symmetry.
>
> Signed-off-by: James Morse <james.morse at arm.com>
>
> ---
> Give me a kick if you want this reworked as a fix (which will then
> conflict with this series), or a backportable version.
>
>  arch/arm64/include/asm/kvm_host.h | 10 ++++++++++
>  virt/kvm/arm/arm.c                |  4 ++++
>  2 files changed, 14 insertions(+)
>
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index e923b58606e2..d3eb79a9ed6b 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -25,6 +25,7 @@
>  #include <linux/types.h>
>  #include <linux/kvm_types.h>
>  #include <asm/cpufeature.h>
> +#include <asm/daifflags.h>
>  #include <asm/kvm.h>
>  #include <asm/kvm_asm.h>
>  #include <asm/kvm_mmio.h>
> @@ -384,4 +385,13 @@ static inline void __cpu_init_stage2(void)
>  		  "PARange is %d bits, unsupported configuration!", parange);
>  }
>  
> +static inline void kvm_arm_vhe_guest_enter(void)
> +{
> +	local_mask_daif();
> +}
> +
> +static inline void kvm_arm_vhe_guest_exit(void)
> +{
> +	local_restore_daif(DAIF_PROCCTX_NOIRQ);
> +}
>  #endif /* __ARM64_KVM_HOST_H__ */
> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> index b9f68e4add71..665529924b34 100644
> --- a/virt/kvm/arm/arm.c
> +++ b/virt/kvm/arm/arm.c
> @@ -698,9 +698,13 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
>  		 */
>  		trace_kvm_entry(*vcpu_pc(vcpu));
>  		guest_enter_irqoff();
> +		if (has_vhe())
> +			kvm_arm_vhe_guest_enter();
>  
>  		ret = kvm_call_hyp(__kvm_vcpu_run, vcpu);
>  
> +		if (has_vhe())
> +			kvm_arm_vhe_guest_exit();
>  		vcpu->mode = OUTSIDE_GUEST_MODE;
>  		vcpu->stat.exits++;
>  		/*

Why is that masking limited to entering/exiting the guest? I would have
though that it would have been put in the kvm_call_hyp helper, in order
to cover all "HYP" accesses. Or is it that you've worked out that only
the guest run actually requires this because none of the other HYP
helpers are changing the flags?

Thanks,

	M.
-- 
Jazz is not dead, it just smell funny.