linux-kernel - Re: [PATCH 3/3] KVM: SVM: allow to intercept all exceptions for debug

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Date:   Thu, 18 Mar 2021 16:35:24 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Wanpeng Li <wanpengli@...cent.com>,
        Kieran Bingham <kbingham@...nel.org>,
        Jessica Yu <jeyu@...nel.org>,
        Jan Kiszka <jan.kiszka@...mens.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Jim Mattson <jmattson@...gle.com>,
        Borislav Petkov <bp@...en8.de>,
        Stefano Garzarella <sgarzare@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH 3/3] KVM: SVM: allow to intercept all exceptions for debug

On Thu, Mar 18, 2021, Joerg Roedel wrote:
> On Thu, Mar 18, 2021 at 11:24:25AM +0200, Maxim Levitsky wrote:
> > But again this is a debug feature, and it is intended to allow the user
> > to shoot himself in the foot.
> 
> And one can't debug SEV-ES guests with it, so what is the point of
> enabling it for them too?

Agreed.  I can see myself enabling debug features by default, it would be nice
to not having to go out of my way to disable them for SEV-ES/SNP guests.

Skipping SEV-ES guests should not be difficult; KVM could probably even
print a message stating that the debug hook is being ignored.  One thought would
be to snapshot debug_intercept_exceptions at VM creation, and simply zero it out
for incompatible guests.  That would also allow changing debug_intercept_exceptions
without reloading KVM, which IMO would be very convenient.