Re: [PATCH 03/10] KVM: x86: Add KVM_[GS]ET_CLOCK_GUEST for accurate KVM clock migration

From: David Woodhouse <dwmw2@infradead.org>
To: paul@xen.org, kvm@vger.kernel.org
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Jonathan Corbet <corbet@lwn.net>,
	 Sean Christopherson <seanjc@google.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	 Shuah Khan <shuah@kernel.org>,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	 linux-kselftest@vger.kernel.org,
	Oliver Upton <oliver.upton@linux.dev>,
	 Marcelo Tosatti <mtosatti@redhat.com>,
	jalliste@amazon.co.uk, sveith@amazon.de
Subject: Re: [PATCH 03/10] KVM: x86: Add KVM_[GS]ET_CLOCK_GUEST for accurate KVM clock migration
Date: Fri, 19 Apr 2024 16:49:14 +0100	[thread overview]
Message-ID: <4cfb711179774c1c47487311d0bd4b0497fb8104.camel@infradead.org> (raw)
In-Reply-To: <9985611d-1e0b-48f3-8eb5-fc3e1279b5a5@xen.org>

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On Fri, 2024-04-19 at 16:40 +0100, Paul Durrant wrote:
> 
> > +        * If KVM_REQ_CLOCK_UPDATE is already pending, or if the
> > hv_clock has
> > +        * never been generated at all, call
> > kvm_guest_time_update() to do so.
> > +        * Might as well use the PVCLOCK_TSC_STABLE_BIT as the
> > check for ever
> > +        * having been written.
> > +        */
> > +       if (kvm_check_request(KVM_REQ_CLOCK_UPDATE, v) ||
> > +           !(hv_clock->flags & PVCLOCK_TSC_STABLE_BIT)) {
> > +               if (kvm_guest_time_update(v))
> > +                       return -EINVAL;
> 
> nit: simple nested if, so you could use &&

Yeah, I frowned at that a bit, and decided I preferred it this way just
to highlight the fact that kvm_guest_time_update() is doing a *thing*.
And then we bail with -EINVAL if that thing fails.

If you stick it all in the if() statement, the code flow is logically
the same but it's just a bit less obvious that there are side-effects
here in the middle of the conditions, and that those side-effects are
actually the *main* point of the statement.

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