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

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Message-ID: <ZAZM7z2O1vV5MZjn@google.com>
Date:   Mon, 6 Mar 2023 12:34:13 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     David Matlack <dmatlack@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.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 Sat, Mar 04, 2023, Wang, Wei W wrote:
> 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.

Ya, just make __ret a bool.  I'm 99% certain I just loosely copied from WARN_ON(),
but missed the !!.