Re: [PATCH v3 0/3] x86/crash: Fix double NMI shootdown bug
From: Tom Lendacky
Date: Tue Nov 15 2022 - 13:34:47 EST
On 11/15/22 10:56, Sean Christopherson wrote:
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
^^^^^^^^^^^
I'll see if I can't get someone to update the psuedo-code comment here.
But, to your original question, as far as I understand, GIF is not
automatically set if SVME is cleared from EFER.
Thanks,
Tom