linux-kernel - Re: [PATCH v2 07/10] KVM: X86: Move write_l1_tsc_offset() logic to common code and rename it

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Message-ID: <YKUzc4WJlxvyzw5B@google.com>
Date:   Wed, 19 May 2021 15:49:07 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Stamatis, Ilias" <ilstam@...zon.com>
Cc:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jmattson@...gle.com" <jmattson@...gle.com>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "mtosatti@...hat.com" <mtosatti@...hat.com>,
        "zamsden@...il.com" <zamsden@...il.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "mlevitsk@...hat.com" <mlevitsk@...hat.com>,
        "wanpengli@...cent.com" <wanpengli@...cent.com>
Subject: Re: [PATCH v2 07/10] KVM: X86: Move write_l1_tsc_offset() logic to
 common code and rename it

On Wed, May 19, 2021, Stamatis, Ilias wrote:
> On Wed, 2021-05-19 at 00:05 +0000, Sean Christopherson wrote:
> > On Wed, May 12, 2021, Ilias Stamatis wrote:
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index 1db6cfc2079f..f3ba1be4d5b9 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -2377,8 +2377,23 @@ EXPORT_SYMBOL_GPL(kvm_set_02_tsc_multiplier);
> > > 
> > >  static void kvm_vcpu_write_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
> > >  {
> > > +     trace_kvm_write_tsc_offset(vcpu->vcpu_id,
> > > +                                vcpu->arch.l1_tsc_offset,
> > > +                                offset);
> > > +
> > >       vcpu->arch.l1_tsc_offset = offset;
> > > -     vcpu->arch.tsc_offset = static_call(kvm_x86_write_l1_tsc_offset)(vcpu, offset);
> > > +     vcpu->arch.tsc_offset = offset;
> > > +
> > > +     if (is_guest_mode(vcpu)) {
> > 
> > Unnecessary curly braces.
> 
> Really? We are supposed to have a 6-lines body without brackets? I'm not
> opposing, I'm just surprised that that's the coding standard.

Comments don't (technically) count.  I usually avoid the ambiguity by putting
the comment above the if statement.  That also helps with indentation, e.g.

	/*
	 * This is a comment.
	 */
	if (is_guest_mode(vcpu))
		kvm_set_02_tsc_offset(vcpu);

> > > +             /*
> > > +              * We're here if L1 chose not to trap WRMSR to TSC and
> > > +              * according to the spec this should set L1's TSC (as opposed
> > > +              * to setting L1's offset for L2).
> > > +              */
> > 
> > While we're shuffling code, can we improve this comment?  It works for the WRMSR
> > case, but makes no sense in the context of host TSC adjustments.  It's not at all
> > clear to me that it's even correct or relevant in those cases.
> > 
> 
> Do you suggest removing it completely or how do you want it to be? I don't
> mind deleting it.

Heh, I'd happily write the comment, except I have no idea what the logic is in
the non-WRMSR case.  I do think we need a comment, IMO none of paths that lead
to changing the TSC offset while L2 is active are obvious.