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

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Date:   Thu, 21 Jul 2022 16:25:40 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     Santosh Shukla <santosh.shukla@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Tom Lendacky <thomas.lendacky@....com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 4/7] KVM: SVM: Report NMI not allowed when Guest busy
 handling VNMI

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...