linux-kernel - Re: [PATCH] KVM: x86/xen: Make number of event channels defines less magical

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Message-ID: <Y3KZVUCCH+YQDbqu@google.com>
Date:   Mon, 14 Nov 2022 19:39:01 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paul Durrant <paul@....org>
Subject: Re: [PATCH] KVM: x86/xen: Make number of event channels defines less
 magical

On Mon, Nov 14, 2022, David Woodhouse wrote:
> On Mon, 2022-11-14 at 18:16 +0000, Sean Christopherson wrote:
> > Use BITS_PER_BYTE and sizeof_field() to compute the number of Xen event
> > channels.  The compat version at least uses sizeof_field(), but the
> > regular version open codes sizeof_field(), BITS_PER_BYTE, and combines
> > literals in the process, which makes it far too difficult to understand
> > relatively straightforward code.
> > 
> > No functional change intended.
> 
> Slightly dubious about changing the regular one, since that's just
> imported directly from Xen public header files.

Ugh.  I worried that might be the case.  An alternative approach to help document
things from a KVM perspective would be something like:

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 93c628d3e3a9..7769f3b98af0 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -1300,6 +1300,9 @@ int kvm_xen_hypercall(struct kvm_vcpu *vcpu)
 
 static inline int max_evtchn_port(struct kvm *kvm)
 {
+       BUILD_BUG_ON(EVTCHN_2L_NR_CHANNELS !=
+                    (sizeof_field(struct shared_info, evtchn_pending) * BITS_PER_BYTE));
+
        if (IS_ENABLED(CONFIG_64BIT) && kvm->arch.xen.long_mode)
                return EVTCHN_2L_NR_CHANNELS;
        else