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

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Message-ID: <4116d6ce75a85faccfe7a2b3967528f0561974ae.camel@redhat.com>
Date:   Tue, 16 Mar 2021 12:51:20 +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 Tue, 2021-03-16 at 09:32 +0100, Joerg Roedel wrote:
> Hi Maxim,
> 
> On Tue, Mar 16, 2021 at 12:10:20AM +0200, Maxim Levitsky wrote:
> > -static int (*const svm_exit_handlers[])(struct kvm_vcpu *vcpu) = {
> > +static int (*svm_exit_handlers[])(struct kvm_vcpu *vcpu) = {
> 
> Can you keep this const and always set the necessary handlers? If
> exceptions are not intercepted they will not be used.
> 
> > @@ -333,7 +334,9 @@ static inline void clr_exception_intercept(struct vcpu_svm *svm, u32 bit)
> >  	struct vmcb *vmcb = svm->vmcb01.ptr;
> >  
> >  	WARN_ON_ONCE(bit >= 32);
> > -	vmcb_clr_intercept(&vmcb->control, INTERCEPT_EXCEPTION_OFFSET + bit);
> > +
> > +	if (!((1 << bit) & debug_intercept_exceptions))
> > +		vmcb_clr_intercept(&vmcb->control, INTERCEPT_EXCEPTION_OFFSET + bit);
> 
> This will break SEV-ES guests, as those will not cause an intercept but
> now start to get #VC exceptions on every other exception that is raised.
> SEV-ES guests are not prepared for that and will not even boot, so
> please don't enable this feature for them.

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).

I have nothing against not allowing this for SEV-ES guests though.
What do you think?


Best regards,
	Maxim Levitsky