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

[Marc Zyngier]

On Sun, 09 May 2021 14:07:45 +0100,
Zenghui Yu <yuzenghui at huawei.com> wrote:
> 
> On 2021/5/6 22:29, Marc Zyngier wrote:
> > On Thu, 06 May 2021 12:43:26 +0100,
> > Zenghui Yu <yuzenghui at huawei.com> wrote:
> >> 
> >> On 2021/5/6 14:33, Marc Zyngier wrote:
> >>> On Wed, 05 May 2021 17:46:51 +0100,
> >>> Marc Zyngier <maz at kernel.org> wrote:
> >>>> 
> >>>> Hi Zenghui,
> >>>> 
> >>>> On Wed, 05 May 2021 15:23:02 +0100,
> >>>> Zenghui Yu <yuzenghui at huawei.com> 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 at kernel.org>
> >>>>> 
> >>>>> [...]
> >>>>> 
> >>>>>> @@ -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.
> 
> I'm sorry for the misleading words.
> 
> The reported problem (secondary vcpu entry point corruption) was noticed
> after a guest reboot. On rebooting, all vcpus will go back to userspace,
> either because of a vcpu PSCI_SYSTEM_RESET call (with a
> KVM_SYSTEM_EVENT_RESET system event in result), or because of a pending
> signal targetting the vcpu thread. Userspace (I used QEMU) will then
> perform the vcpu reset using the KVM_ARM_VCPU_INIT ioctl, of course!

OK, that's exactly the scenario I had in mind, and we are in violent
agreement! :-)

> WFI is the last instruction executed by the secondary vcpu before
> rebooting. Emulating it results in a PC-altering flag.
> 
> What I was going to say is that maybe we can reproduce this problem with
> a much simpler userspace program (not QEMU, no reboot) -- perform vcpu
> reset while the vcpu is concurrently executing WFI, and see if the
> result PC is set to 0 (per the KVM_ARM_VCPU_INIT doc). Maybe we can
> achieve it with a kvm selftest case but "I hadn't tried", which turned
> out to be misleading.

No worries. At least I know we have the same understanding of the
problem and we can look at the solution.

> I'll have a look at your branch.

Thanks,

	M.

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