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

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

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.

quoted

This was noticed with a latest guest kernel, at least with commit
dccc9da22ded ("arm64: Improve parking of stopped CPUs"), which put the
stopped VCPUs in the WFx loop. The guest kernel shouted at me that

	"CPU: CPUs started in inconsistent modes"

Ah, the perks of running guests with "quiet"... Well caught.

quoted

*after* rebooting. The problem is that the secondary entry point was
corrupted by KVM as explained above. All of the secondary processors
started from set_cpu_boot_mode_flag(), with w0=0. Oh well...

I write the below diff and guess it will help. But I have to look at all
other places where we adjust PC directly to make a right fix. Please let
me know what do you think.


Thanks,
Zenghui

---->8----

diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
index 956cdc240148..ed647eb387c3 100644
--- a/arch/arm64/kvm/reset.c
+++ b/arch/arm64/kvm/reset.c

@@ -265,7 +265,12 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
 		if (vcpu->arch.reset_state.be)
 			kvm_vcpu_set_be(vcpu);

+		/*
+		 * Don't bother with the KVM_ARM64_INCREMENT_PC flag while
+		 * using this version of __adjust_pc().
+		 */
 		*vcpu_pc(vcpu) = target_pc;
+		vcpu->arch.flags &= ~KVM_ARM64_INCREMENT_PC;

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.

I'll try and cook something up.

Thanks,

	M.

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

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel