linux-kernel - Re: [PATCH v2 0/2] kvm: x86: Convey the exit reason to user-space on emulation failure

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Message-ID: <YOh0gNxRJ67Lbo7g@google.com>
Date:   Fri, 9 Jul 2021 16:08:32 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Matlack <dmatlack@...gle.com>
Cc:     David Edmondson <dme@....org>, linux-kernel@...r.kernel.org,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Borislav Petkov <bp@...en8.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 0/2] kvm: x86: Convey the exit reason to user-space on
 emulation failure

On Thu, Jul 08, 2021, David Matlack wrote:
> On Thu, Jul 08, 2021 at 09:13:38PM +0100, David Edmondson wrote:
> > On Thursday, 2021-07-08 at 18:38:18 UTC, David Matlack wrote:
> > > On Thu, Jul 08, 2021 at 03:17:40PM +0100, David Edmondson wrote:
> > >> I can't cite an example of where this has definitively led in a
> > >> direction that helped solve a problem, but we do sometimes see emulation
> > >> failures reported in situations where we are not able to reproduce the
> > >> failures on demand and the existing information provided at the time of
> > >> failure is either insufficient or suspect.
> > >> 
> > >> Given that, I'm left casting about for data that can be made available
> > >> to assist in postmortem analysis of the failures.
> > >
> > > Understood, thanks for the context. My only concern would be that
> > > userspace APIs are difficult to change once they exist.
> > 
> > Agreed.
> > 
> > > If it turns out knowing the exit reason does not help with debugging
> > > emulation failures we'd still be stuck with exporting it on every
> > > emulation failure.

I can think of multiple cases where knowing why KVM emulated in the first place
would be helpful, e.g. a failure on EPT misconfig (MMIO) exit could be a simple
"drat, KVM doesn't handle SSE instructions", whereas a failure on a descriptor
table exit (for UMIP emulation) would be a completely different mess.