linux-kernel - RE: [PATCH v1] KVM: allow KVM_BUG/KVM_BUG

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Date:   Sat, 4 Mar 2023 04:25:00 +0000
From:   "Wang, Wei W" <wei.w.wang@...el.com>
To:     David Matlack <dmatlack@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>
CC:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v1] KVM: allow KVM_BUG/KVM_BUG_ON to handle 64-bit cond

On Saturday, March 4, 2023 1:36 AM, David Matlack wrote:
> > > On Friday, March 3, 2023 2:12 AM, Mingwei Zhang wrote:
> > > > > On Thursday, March 2, 2023 12:55 PM, Mingwei Zhang wrote:
> > > > > > I don't get it. Why bothering the type if we just do this?
> > > > > >
> > > > > > diff --git a/include/linux/kvm_host.h
> > > > > > b/include/linux/kvm_host.h index 4f26b244f6d0..10455253c6ea
> > > > > > 100644
> > > > > > --- a/include/linux/kvm_host.h
> > > > > > +++ b/include/linux/kvm_host.h
> > > > > > @@ -848,7 +848,7 @@ static inline void kvm_vm_bugged(struct
> > > > > > kvm
> > > > > > *kvm)
> > > > > >
> > > > > >  #define KVM_BUG(cond, kvm, fmt...)                           \
> > > > > >  ({                                                           \
> > > > > > -     int __ret = (cond);                                     \
> > > > > > +     int __ret = !!(cond);                                   \
> > > > >
> > > > > This is essentially "bool __ret". No biggie to change it this way.
> > > >
> > > > !! will return an int, not a boolean, but it is used as a boolean.
> > >
> > > What's the point of defining it as an int when actually being used as a
> Boolean?
> > > Original returning of an 'int' is a bug in this sense. Either
> > > returning a Boolean or the same type (length) as cond is good way to me.
> >
> > What's the point of using an integer? I think we need to ask the
> > original author. But I think one of the reasons might be convenience
> > as the return value. I am not sure if we can return a boolean in the
> > function. But it should be fine here since it is a macro.
> >
> > Anyway, returning an 'int' is not a bug. The bug is the casting from
> > 'cond' to the integer that may lose information and this is what you
> > have captured.
> 
> typeof() won't work if cond is a bitfield. See commit 8d4fbcfbe0a4 ("Fix
> WARN_ON() on bitfield ops") from Linus from back in 2007:

Yes, this seems to be a good reason for not going for typeof. Thanks for sharing.

> 
> As for int versus bool, I don't see a strong argument for either. So let's stick
> with int since that's what the current code is using and that aligns with the
> generic kernel WARN_ON().
> 
> If someone wants to propose using a bool instead of an int that should be a
> separate commit anyway and needs an actual justification.

Wait a bit. Let me seek for a valid reason from the WARN side first.

CodingStyle already says:
bool function return types and stack variables are always fine to use whenever
appropriate. Use of bool is encouraged to improve readability and is often a
better option than 'int' for storing boolean values.