Hi Ladi，
2017-04-25 22:42 GMT+08:00 Ladi Prosek <lprosek@xxxxxxxxxx>:
> 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>
[snip]
>
> - 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;
This will clear SMM related flags for ctxt->emul_flags, so why
overwrites arch.hflags with ctxt->emul_flags matters in
x86_emulate_instruction? In addition, nmi mask is cleared in the above
codes, I didn't find where can set nmi mask again due to SMM unless
enter SMM.
Regards,
Wanpeng Li
