Re: [PATCH v3 0/3] x86/crash: Fix double NMI shootdown bug

[Sean Christopherson <seanjc@google.com>]

On Tue, Nov 15, 2022, Andrew Cooper wrote:
> On 14/11/2022 23:34, Sean Christopherson wrote:
> > Tom,
> >
> > I Cc'd you this time around because the APM doesn't explicitly state that
> > GIF is set when EFER.SVME is disabled.  KVM's nSVM emulation does set GIF
> > in this case, but it's not clear whether or not KVM is making up behavior.
> > If clearing EFER.SVME doesn't set GIF, then patch 1 needs to be modified
> > to try STGI before clearing EFER.SVME, e.g. if a crash is initiated from
> > KVM between CLGI and STGI.  Responding CPUs are "safe" because GIF=0 also
> > blocks NMIs, but the initiating CPU might leave GIF=0 when jumping into
> > the new kernel.
> 
> GIF exists independently of EFER.SVME.
> 
> It is also gets set by the SKINIT instruction, which is why there is an
> asymmetry on the #UD conditions of STGI and CLGI.
> 
> STGI is also intended to be used by the DLME once critical
> initialisation is done, hence why it's dependent on EFER.SVME || SKINIT.

Gah, stupid APM.  The pseudocode states "EFER.SVME || CPUID.SKINIT", but the
description and the comment both say that SVM must be enabled to execute SKINIT,
which made me hope that disabling SVM would reset everything.

  This instruction generates a #UD exception if SVM is not enabled. See “Enabling
  SVM” in AMD64 Architecture Programmer’s Manual Volume 2: System Instructions,
  order# 24593.

...

  IF ((EFER.SVME == 0) && !(CPUID 8000_0001.ECX[SKINIT]) || (!PROTECTED_MODE))
          EXCEPTION [#UD]  // This instruction can only be executed
                           // in protected mode with SVM enabled
                                                     ^^^^^^^^^^^