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

[Sean Christopherson <seanjc@google.com>]

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.

> > 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.