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Message-ID: <7169229dde171c8e10fb276ff8e1a869af99b39d.camel@redhat.com>
Subject: Re: [PATCH 3/3] KVM: SVM: allow to intercept all exceptions for
 debug
From:   Maxim Levitsky <mlevitsk@redhat.com>
To:     Sean Christopherson <seanjc@google.com>,
        Joerg Roedel <joro@8bytes.org>
Cc:     kvm@vger.kernel.org, Vitaly Kuznetsov <vkuznets@redhat.com>,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Wanpeng Li <wanpengli@tencent.com>,
        Kieran Bingham <kbingham@kernel.org>,
        Jessica Yu <jeyu@kernel.org>,
        Jan Kiszka <jan.kiszka@siemens.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        Jim Mattson <jmattson@google.com>,
        Borislav Petkov <bp@alien8.de>,
        Stefano Garzarella <sgarzare@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@suse.de>
Date:   Thu, 18 Mar 2021 18:41:57 +0200
In-Reply-To: <YFOBTITk7EkGdzR2@google.com>
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         <4116d6ce75a85faccfe7a2b3967528f0561974ae.camel@redhat.com>
         <YFMbLWLlGgbOJuN/@8bytes.org>
         <8ba6676471dc8c8219e35d6a1695febaea20bb0b.camel@redhat.com>
         <YFN2HGG7ZTdamM7k@8bytes.org> <YFOBTITk7EkGdzR2@google.com>
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On Thu, 2021-03-18 at 16:35 +0000, Sean Christopherson wrote:
> 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?
You can create a special SEV-ES guest which does handle all exceptions via
#VC, or just observe it fail which can be useful for some whatever reason.
> 
> 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.
This does sound like a valid reason to disable this for SEV-ES.

> 
> 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.
> 
So all right I'll disable this for SEV-ES. 
The idea to change the debug_intercept_exceptions on the fly is also a good idea,
I will implement it in next version of the patches.

Thanks for the review,
Best regards,
	Maxim Levitsky