Re: [PATCH v2 03/11] KVM: arm64: Make kvm_skip_instr() and co private to HYP

On Thu, 06 May 2021 12:43:26 +0100,
Zenghui Yu [off-list ref] wrote:

On 2021/5/6 14:33, Marc Zyngier wrote:

quoted

On Wed, 05 May 2021 17:46:51 +0100,
Marc Zyngier [off-list ref] wrote:

quoted

Hi Zenghui,

On Wed, 05 May 2021 15:23:02 +0100,
Zenghui Yu [off-list ref] wrote:

quoted

Hi Marc,

On 2020/11/3 0:40, Marc Zyngier wrote:

quoted

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>

[...]

quoted

@@ -133,6 +134,8 @@ static int __kvm_vcpu_run_vhe(struct kvm_vcpu *vcpu)
 	__load_guest_stage2(vcpu->arch.hw_mmu);
 	__activate_traps(vcpu);
+	__adjust_pc(vcpu);

If the INCREMENT_PC flag was set (e.g., for WFx emulation) while we're
handling PSCI CPU_ON call targetting this VCPU, the *target_pc* (aka
entry point address, normally provided by the primary VCPU) will be
unexpectedly incremented here. That's pretty bad, I think.

How can you online a CPU using PSCI if that CPU is currently spinning
on a WFI? Or is that we have transitioned via userspace to perform the
vcpu reset? I can imagine it happening in that case.

I hadn't tried to reset VCPU from userspace. That would be a much easier
way to reproduce this problem.

Then I don't understand how you end-up there. If the vcpu was in WFI,
it wasn't off and PSCI_CPU_ON doesn't have any effect.

quoted

Actually, this is far worse than it looks, and this only papers over
one particular symptom. We need to resolve all pending PC updates
*before* returning to userspace, or things like live migration can
observe an inconsistent state.

Ah yeah, agreed.

Apart from the PC manipulation, I noticed that when handling the user
GET_VCPU_EVENTS request:

|	/*
|	 * We never return a pending ext_dabt here because we deliver it to
|	 * the virtual CPU directly when setting the event and it's no longer
|	 * 'pending' at this point.
|	 */

Which isn't true anymore now that we defer the exception injection right
before the VCPU entry.

I believe the comment will be valid again once I fix the core issue,
which is that we shouldn't return to userspace with pending PC
adjustments. As long as KVM_GET_VCPU_EVENTS isn't issued on a running
vcpu (which looks pointless to me), this should be just fine.

Thanks,

	M.

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

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