[PATCH v4 09/21] KVM: arm/arm64: mask/unmask daif around VHE guests

[james.morse@arm.com (James Morse)]

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@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..a0e2f7962401 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_daif_mask();
+}
+
+static inline void kvm_arm_vhe_guest_exit(void)
+{
+	local_daif_restore(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++;
 		/*
-- 
2.13.3