[PATCH v3 07/28] KVM: arm64: Don't evaluate HCR_EL2.NV nor HFGITR_EL2.ERET on ERET fast path

[Marc Zyngier]

We currently avoid using the ERET fast path if the guest has HCR_EL2.NV
set. This is an odd check, as NV doesn't mean much if HCR_EL2.TGE==1.

Similarly, evaluating HFGITR_EL2.ERET makes little sense, as
this only applies to the nested context, while the ERET fast-path
is purely for the benefit of L1.

Replace these bizarre checks with is_nested_ctxt() which makes a lot
more sense: if we are running an L2, the ERET trap must go to L1.

Fixes: dd0717a998f77 ("KVM: arm64: nv: Fast-track 'InHost' exception returns")
Signed-off-by: Marc Zyngier <maz at kernel.org>
---
 arch/arm64/kvm/hyp/vhe/switch.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
index bbe9cebd3d9d5..684cebf23aa0e 100644
--- a/arch/arm64/kvm/hyp/vhe/switch.c
+++ b/arch/arm64/kvm/hyp/vhe/switch.c
@@ -344,13 +344,15 @@ static bool kvm_hyp_handle_eret(struct kvm_vcpu *vcpu, u64 *exit_code)
 	 * if this is a VHE guest hypervisor returning to its own
 	 * userspace, or the hypervisor performing a local exception
 	 * return. No need to save/restore registers, no need to
-	 * switch S2 MMU. Just do the canonical ERET.
+	 * switch S2 MMU. Just do the canonical ERET unless we are in
+	 * nested context.
 	 *
-	 * Unless the trap has to be forwarded further down the line,
-	 * of course...
+	 * Note that this is made possible because KVM itself never traps
+	 * ERET when running an L2. The consequence is that any ERET trap is
+	 * the result of HCR_EL2 or HFGITR_EL2 programming by L1 for its own
+	 * guest, and the exception must be forwarded to L1.
 	 */
-	if ((__vcpu_sys_reg(vcpu, HCR_EL2) & HCR_NV) ||
-	    (__vcpu_sys_reg(vcpu, HFGITR_EL2) & HFGITR_EL2_ERET))
+	if (is_nested_ctxt(vcpu))
 		return false;
 
 	spsr = read_sysreg_el1(SYS_SPSR);
-- 
2.47.3