linux-kernel - Re: [PATCH v2 06/13] KVM: x86: Refactor emulate tracepoint to explicitly take context

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Date:   Tue, 3 Mar 2020 09:42:42 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     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
Subject: Re: [PATCH v2 06/13] KVM: x86: Refactor emulate tracepoint to
 explicitly take context

On Tue, Mar 03, 2020 at 06:29:30PM +0100, Paolo Bonzini wrote:
> On 03/03/20 17:48, Sean Christopherson wrote:
> >>>  	TP_fast_assign(
> >>>  		__entry->csbase = kvm_x86_ops->get_segment_base(vcpu, VCPU_SREG_CS);
> >> This seems the only usage of 'vcpu' parameter now; I checked and even
> >> after switching to dynamic emulation context allocation we still set
> >> ctxt->vcpu in alloc_emulate_ctxt(), can we get rid of 'vcpu' parameter
> >> here then (and use ctxt->vcpu instead)?
> > Hmm, ya, not sure what I was thinking here.
> > 
> 
> As long as we have one use of vcpu, I'd rather skip this patch and
> adjust patch 8 to use "->".  Even the other "explicitly take context"
> parts are kinda debatable since you still have to do emul_to_vcpu.
> Throwing a handful of
> 
> - 	struct x86_emulate_ctxt *ctxt = &vcpu->arch.emulate_ctxt;
> + 	struct x86_emulate_ctxt *ctxt = vcpu->arch.emulate_ctxt;
> 
> into patch 8 is not bad at all and limits the churn.

Hmm, I'd prefer to explicitly pass @ctxt, even for the tracepoint.  I get
that it's technically unnecessary churn, but explicitly passing @ctxt means
that every funcition that grabs arch.emulate_ctxt (all three of 'em) checks
for a NULL ctxt.  That makes it trivial to visually audit that there's no
risk of a bad pointer dereference, and IMO having @ctxt in the prototype
is helpful to see "oh, this helper is called from within the emulator".