On 25.04.2017 16:42, Ladi Prosek wrote:
> On AMD, the effect of set_nmi_mask called by emulate_iret_real and em_rsm
> on hflags is reverted later on in x86_emulate_instruction where hflags are
> overwritten with ctxt->emul_flags (the kvm_set_hflags call). This manifests
> as a hang when rebooting Windows VMs with QEMU, OVMF, and >1 vcpu.
>
> Instead of trying to merge ctxt->emul_flags into vcpu->arch.hflags after
> an instruction is emulated, this commit deletes emul_flags altogether and
> makes the emulator access vcpu->arch.hflags using two new accessors. This
> way all changes, on the emulator side as well as in functions called from
> the emulator and accessing vcpu state with emul_to_vcpu, are preserved.
>
> More details on the bug and its manifestation with Windows and OVMF:
>
> It's a KVM bug in the interaction between SMI/SMM and NMI, specific to AMD.
> I believe that the SMM part explains why we started seeing this only with
> OVMF.
>
> KVM masks and unmasks NMI when entering and leaving SMM. When KVM emulates
> the RSM instruction in em_rsm, the set_nmi_mask call doesn't stick because
> later on in x86_emulate_instruction we overwrite arch.hflags with
> ctxt->emul_flags, effectively reverting the effect of the set_nmi_mask call.
> The AMD-specific hflag of interest here is HF_NMI_MASK.
>
> When rebooting the system, Windows sends an NMI IPI to all but the current
> cpu to shut them down. Only after all of them are parked in HLT will the
> initiating cpu finish the restart. If NMI is masked, other cpus never get
> the memo and the initiating cpu spins forever, waiting for
> hal!HalpInterruptProcessorsStarted to drop. That's the symptom we observe.
>
> Fixes: a584539b24b8 ("KVM: x86: pass the whole hflags field to emulator and back")
> Signed-off-by: Ladi Prosek <lprosek@xxxxxxxxxx>
[...]
>
> - if ((ctxt->emul_flags & X86EMUL_SMM_INSIDE_NMI_MASK) == 0)
> + if ((ctxt->ops->get_hflags(ctxt) & X86EMUL_SMM_INSIDE_NMI_MASK) == 0)
> ctxt->ops->set_nmi_mask(ctxt, false);
>
> - ctxt->emul_flags &= ~X86EMUL_SMM_INSIDE_NMI_MASK;
> - ctxt->emul_flags &= ~X86EMUL_SMM_MASK;
> + ctxt->ops->set_hflags(ctxt, ctxt->ops->get_hflags(ctxt) &
> + ~X86EMUL_SMM_INSIDE_NMI_MASK &
> + ~X86EMUL_SMM_MASK);
Wonder if that would look better with
& ~(X86EMUL_SMM_INSIDE_NMI_MASK | X86EMUL_SMM_MASK)
[...]
>
> static void toggle_interruptibility(struct kvm_vcpu *vcpu, u32 mask)
> @@ -5314,7 +5326,6 @@ static void init_emulate_ctxt(struct kvm_vcpu *vcpu)
> BUILD_BUG_ON(HF_GUEST_MASK != X86EMUL_GUEST_MASK);
> BUILD_BUG_ON(HF_SMM_MASK != X86EMUL_SMM_MASK);
> BUILD_BUG_ON(HF_SMM_INSIDE_NMI_MASK != X86EMUL_SMM_INSIDE_NMI_MASK);
> - ctxt->emul_flags = vcpu->arch.hflags;
>
> init_decode_cache(ctxt);
> vcpu->arch.emulate_regs_need_sync_from_vcpu = false;
> @@ -5718,8 +5729,6 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
> unsigned long rflags = kvm_x86_ops->get_rflags(vcpu);
> toggle_interruptibility(vcpu, ctxt->interruptibility);
> vcpu->arch.emulate_regs_need_sync_to_vcpu = false;
> - if (vcpu->arch.hflags != ctxt->emul_flags)
> - kvm_set_hflags(vcpu, ctxt->emul_flags);
I like to see that go.
> kvm_rip_write(vcpu, ctxt->eip);
> if (r == EMULATE_DONE)
> kvm_vcpu_check_singlestep(vcpu, rflags, &r);
>
Looks very good to me!
Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
--
Thanks,
David
