Re: [PATCH] KVM: x86: preserve interrupt shadow across SMM entries

Maxim Levitsky
2022-06-08T09:28:02Z

On Tue, 2022-06-07 at 19:22 +0000, Sean Christopherson wrote:
> On Tue, Jun 07, 2022, Paolo Bonzini wrote:
> > On 6/7/22 17:16, Maxim Levitsky wrote:
> > > If the #SMI happens while the vCPU is in the interrupt shadow,
> > > (after STI or MOV SS),
> > > we must both clear it to avoid VM entry failure on VMX,
> > > due to consistency check vs EFLAGS.IF which is cleared on SMM entries,
> > > and restore it on RSM so that #SMI is transparent to the non SMM code.
> > >
> > > To support migration, reuse upper 4 bits of
> > > 'kvm_vcpu_events.interrupt.shadow' to store the smm interrupt shadow.
> > >
> > > This was lightly tested with a linux guest and smm load script,
> > > and a unit test will be soon developed to test this better.
> > >
> > > For discussion: there are other ways to fix this issue:
> > >
> > > 1. The SMM shadow can be stored in SMRAM at some unused
> > > offset, this will allow to avoid changes to kvm_vcpu_ioctl_x86_set_vcpu_events
> >
> > Yes, that would be better (and would not require a new cap).
>
> At one point do we chalk up SMM emulation as a failed experiment and deprecate
> support? There are most definitely more bugs lurking in KVM's handling of
> save/restore across SMI+RSM.

I also kind of agree that SMM was kind of a mistake but these days VMs with secure
boot use it, so we can't stop supporting this.

So do you also agree that I write the interrupt shadow to smram?

Best regards,
Maxim Levitsky

>
> > > 2. #SMI can instead be blocked while the interrupt shadow is active,
> > > which might even be what the real CPU does, however since neither VMX
> > > nor SVM support SMM window handling, this will involve single stepping
> > > the guest like it is currently done on SVM for the NMI window in some cases.
>
> FWIW, blocking SMI in STI/MOVSS shadows is explicitly allowed by the Intel SDM.
> IIRC, modern Intel CPUs block SMIs in MOVSS shadows but not STI shadows.
>