linux-kernel - RE: [PATCH 43/61] KVM: x86: Use KVM cpu caps to mark CR4.LA57 as not-reserved

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Message-ID: <5c0282a66ae54d36af674a568b58071b@AcuMS.aculab.com>
Date:   Tue, 25 Feb 2020 15:19:14 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Paolo Bonzini' <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
CC:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 43/61] KVM: x86: Use KVM cpu caps to mark CR4.LA57 as
 not-reserved

From: Paolo Bonzini
> Sent: 25 February 2020 15:12
> On 24/02/20 23:08, Vitaly Kuznetsov wrote:
> >> +
> >> +static __always_inline bool kvm_cpu_cap_has(unsigned x86_feature)
> >> +{
> >> +	return kvm_cpu_cap_get(x86_feature);
> >> +}
> > I know this works (and I even checked C99 to make sure that it works not
> > by accident) but I have to admit that explicit '!!' conversion to bool
> > always makes me feel safer :-)
> 
> Same here, I don't really like the automagic bool behavior...

I just dislike 'bool'.

Conversion of 0/non-zero to 0/1 isn't completely free.
And something has to 'give' when the referenced memory location
doesn't contain 0 or 1.

One very old version of gcc made a complete hash of:
	bool_var |= function_returning_bool();

I'm not sure what the standard requires nor what current gcc
generates - but you want a 'logical or' instruction.

	David

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