Re: [PATCHv2 4/7] KVM: SVM: Report NMI not allowed when Guest busy handling VNMI

[Sean Christopherson]

On Thu, Jul 21, 2022, Maxim Levitsky wrote:
> On Thu, 2022-07-21 at 16:08 +0000, Sean Christopherson wrote:
> > So we have a poor man's NMI-window exiting.
> 
> Yep, we also intercept IRET for the same purpose, and RSM interception
> is also a place the NMI are evaluated.
> 
> We only single step over the IRET, because NMIs are unmasked _after_ the IRET
> retires.

Heh, check out this blurb from Intel's SDM:

  An execution of the IRET instruction unblocks NMIs even if the instruction
  causes a fault. For example, if the IRET instruction executes with EFLAGS.VM = 1
  and IOPL of less than 3, a general-protection exception is generated (see
  Section 20.2.7, “Sensitive Instructions”). In such a case, NMIs are unmasked
  before the exception handler is invoked.

Not that I want to try and handle that in KVM if AMD follows suit, I simply find
it amusing how messy this all is.  A true NMI-window exit would have been nice...