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

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Date:   Thu, 18 Mar 2021 11:24:25 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     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>,
        Sean Christopherson <seanjc@...gle.com>,
        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, 2021-03-18 at 10:19 +0100, Joerg Roedel wrote:
> On Tue, Mar 16, 2021 at 12:51:20PM +0200, Maxim Levitsky wrote:
> > I agree but what is wrong with that? 
> > This is a debug feature, and it only can be enabled by the root,
> > and so someone might actually want this case to happen
> > (e.g to see if a SEV guest can cope with extra #VC exceptions).
> 
> That doesn't make sense, we know that and SEV-ES guest can't cope with
> extra #VC exceptions, so there is no point in testing this. It is more a
> way to shot oneself into the foot for the user and a potential source of
> bug reports for SEV-ES guests.

But again this is a debug feature, and it is intended to allow the user
to shoot himself in the foot. Bug reports for a debug feature
are autoclosed. It is no different from say poking kernel memory with
its built-in gdbstub, for example.

Best regards,
	Maxim Levitsky

> 
> 
> > I have nothing against not allowing this for SEV-ES guests though.
> > What do you think?
> 
> I think SEV-ES guests should only have the intercept bits set which
> guests acutally support

> 
> Regards,
> 
> 	Joerg
>