Hi Zenghui,
On Wed, 05 May 2021 15:23:02 +0100,
Zenghui Yu <yuzenghui@xxxxxxxxxx> wrote:
>
> Hi Marc,
>
> On 2020/11/3 0:40, 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@xxxxxxxxxx>
>
> [...]
>
> > @@ -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.
> 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.
> *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;
I think you need to make it a lot stronger: any PC-altering flag will
do the wrong thing here. I'd go and clear all the exception bits:
Thanks,
M.
diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
index 956cdc240148..54913612d602 100644
--- a/arch/arm64/kvm/reset.c
+++ b/arch/arm64/kvm/reset.c
@@ -265,6 +265,12 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
if (vcpu->arch.reset_state.be)
kvm_vcpu_set_be(vcpu);
+ /*
+ * We're reseting the CPU, make sure there is no
+ * pending exception or other PC-altering event.
+ */
+ vcpu->arch.flags &= ~(KVM_ARM64_PENDING_EXCEPTION |
+ KVM_ARM64_EXCEPT_MASK);
*vcpu_pc(vcpu) = target_pc;
vcpu_set_reg(vcpu, 0, vcpu->arch.reset_state.r0);
--
Without deviation from the norm, progress is not possible.
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