On Mon, Oct 26, 2020 at 01:34:42PM +0000, Marc Zyngier wrote:

In an effort to remove the vcpu PC manipulations from EL1 on nVHE
systems, move kvm_skip_instr() to be HYP-specific. EL1's intent
to increment PC post emulation is now signalled via a flag in the
vcpu structure.

Signed-off-by: Marc Zyngier <maz@kernel.org>

[...]

+/*
+ * Adjust the guest PC on entry, depending on flags provided by EL1
+ * for the purpose of emulation (MMIO, sysreg).
+ */
+static inline void __adjust_pc(struct kvm_vcpu *vcpu)
+{
+	if (vcpu->arch.flags & KVM_ARM64_INCREMENT_PC) {
+		kvm_skip_instr(vcpu);
+		vcpu->arch.flags &= ~KVM_ARM64_INCREMENT_PC;
+	}
+}

What's your plan for restricting *when* EL1 can ask for the PC to be
adjusted?

I'm assuming that either:

1. You have EL2 sanity-check all responses from EL1 are permitted for
   the current state. e.g. if EL1 asks to increment the PC, EL2 must
   check that that was a sane response for the current state.

2. You raise the level of abstraction at the EL2/EL1 boundary, such that
   EL2 simply knows. e.g. if emulating a memory access, EL1 can either
   provide the response or signal an abort, but doesn't choose to
   manipulate the PC as EL2 will infer the right thing to do.

I know that either are tricky in practice, so I'm curious what your view
is. Generally option #2 is easier to fortify, but I guess we might have
to do #1 since we also have to support unprotected VMs?

Thanks,
Mark.

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