Subject: Re: [PATCH v3 23/27] KVM: arm64: nv: Add SVC trap forwarding

From: Marc Zyngier <[email-masked]>
Date: Thu, 10 Aug 2023 11:42:19 +0100

Hi Eric,

On Thu, 10 Aug 2023 09:35:41 +0100,
Eric Auger <[email-masked]> wrote:
> 
> Hi Marc,
> 
> On 8/8/23 13:47, Marc Zyngier wrote:
> > HFGITR_EL2 allows the trap of SVC instructions to EL2. Allow these
> > traps to be forwarded. Take this opportunity to deny any 32bit activity
> > when NV is enabled.
> 
> I can't figure out how HFGITR_EL2.{SVC_EL1, SVC_EL0 and ERET} are
> handled. Please could you explain.

- SVC: KVM itself never traps it, so any trap of SVC must be the
  result of a guest trap -- we don't need to do any demultiplexing. We
  thus directly inject the trap back. This is what the comment in
  handle_svc() tries to capture, but obviously fails to convey the
  point.

- ERET: This is already handled since 6898a55ce38c ("KVM: arm64: nv:
  Handle trapped ERET from virtual EL2"). Similarly to SVC, KVM never
  traps it unless we run NV.

Now, looking into it, I think I'm missing the additional case where
the L2 guest runs at vEL1. I'm about to add the following patchlet:

diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
index 3b86d534b995..617ae6dea5d5 100644
--- a/arch/arm64/kvm/handle_exit.c
+++ b/arch/arm64/kvm/handle_exit.c
@@ -222,7 +222,22 @@ static int kvm_handle_eret(struct kvm_vcpu *vcpu)
 	if (kvm_vcpu_get_esr(vcpu) & ESR_ELx_ERET_ISS_ERET)
 		return kvm_handle_ptrauth(vcpu);
 
-	kvm_emulate_nested_eret(vcpu);
+	/*
+	 * If we got here, two possibilities:
+	 *
+	 * - the guest is in EL2, and we need to fully emulate ERET
+	 *
+	 * - the guest is in EL1, and we need to reinject the
+         *   exception into the L1 hypervisor.
+	 *
+	 * If KVM ever traps ERET for its own use, we'll have to
+	 * revisit this.
+	 */
+	if (is_hyp_ctxt(vcpu))
+		kvm_emulate_nested_eret(vcpu);
+	else
+		kvm_inject_nested_sync(vcpu, kvm_vcpu_get_esr(vcpu));
+
 	return 1;
 }

Does the above help?

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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