linux-kernel - Re: [PATCH v2 13/21] KVM: x86: Formalize blocking of nested pending exceptions

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Date:   Wed, 06 Jul 2022 23:03:24 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Oliver Upton <oupton@...gle.com>,
        Peter Shier <pshier@...gle.com>
Subject: Re: [PATCH v2 13/21] KVM: x86: Formalize blocking of nested pending
 exceptions

On Wed, 2022-07-06 at 17:36 +0000, Sean Christopherson wrote:
> On Wed, Jul 06, 2022, Maxim Levitsky wrote:
> > On Tue, 2022-06-14 at 20:47 +0000, Sean Christopherson wrote:
> > > Capture nested_run_pending as block_pending_exceptions so that the logic
> > > of why exceptions are blocked only needs to be documented once instead of
> > > at every place that employs the logic.
> > > 
> > > No functional change intended.
> > > 
> > > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > > ---
> > >  arch/x86/kvm/svm/nested.c | 20 ++++++++++----------
> > >  arch/x86/kvm/vmx/nested.c | 23 ++++++++++++-----------
> > >  2 files changed, 22 insertions(+), 21 deletions(-)
> > > 
> > > diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> > > index 471d40e97890..460161e67ce5 100644
> > > --- a/arch/x86/kvm/svm/nested.c
> > > +++ b/arch/x86/kvm/svm/nested.c
> > > @@ -1347,10 +1347,16 @@ static inline bool nested_exit_on_init(struct vcpu_svm *svm)
> > >  
> > >  static int svm_check_nested_events(struct kvm_vcpu *vcpu)
> > >  {
> > > -	struct vcpu_svm *svm = to_svm(vcpu);
> > > -	bool block_nested_events =
> > > -		kvm_event_needs_reinjection(vcpu) || svm->nested.nested_run_pending;
> > >  	struct kvm_lapic *apic = vcpu->arch.apic;
> > > +	struct vcpu_svm *svm = to_svm(vcpu);
> > > +	/*
> > > +	 * Only a pending nested run blocks a pending exception.  If there is a
> > > +	 * previously injected event, the pending exception occurred while said
> > > +	 * event was being delivered and thus needs to be handled.
> > > +	 */
> > 
> > Tiny nitpick about the comment:
> > 
> > One can say that if there is an injected event, this means that we
> > are in the middle of handling it, thus we are not on instruction boundary,
> > and thus we don't process events (e.g interrupts).
> > 
> > So maybe write something like that?
> 
> Hmm, that's another way to look at things.  My goal with the comment was to try
> and call out that any pending exception is a continuation of the injected event,
> i.e. that the injected event won't be lost.  Talking about instruction boundaries
> only explains why non-exception events are blocked, it doesn't explain why exceptions
> are _not_ blocked.
> 
> I'll add a second comment above block_nested_events to capture the instruction
> boundary angle.
> 
> > > +	bool block_nested_exceptions = svm->nested.nested_run_pending;
> > > +	bool block_nested_events = block_nested_exceptions ||
> > > +				   kvm_event_needs_reinjection(vcpu);
> > 
> > Tiny nitpick: I don't like that much the name 'nested' as
> > it can also mean a nested exception (e.g exception that
> > happened while jumping to an exception  handler).
> > 
> > Here we mean just exception/events for the guest, so I would suggest
> > to just drop the word 'nested'.
> 
> I don't disagree, but I'd prefer to keep the current naming because the helper
> itself is *_check_nested_events().  I'm not opposed to renaming things in the
> future, but I don't want to do that in this series.
> 
Yep, makes sense.

Best regards,
	Maxim Levitsky