linux-kernel - Re: [PATCH 3/3] KVM: s390: Increase size of union sca

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Message-ID: <20241125171705.46e64cef@p-imbrenda>
Date: Mon, 25 Nov 2024 17:17:05 +0100
From: Claudio Imbrenda <imbrenda@...ux.ibm.com>
To: Heiko Carstens <hca@...ux.ibm.com>
Cc: Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank
 <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] KVM: s390: Increase size of union sca_utility to
 four bytes

On Mon, 25 Nov 2024 14:40:22 +0100
Heiko Carstens <hca@...ux.ibm.com> wrote:

> On Mon, Nov 25, 2024 at 01:20:42PM +0100, Claudio Imbrenda wrote:
> > On Mon, 25 Nov 2024 12:50:39 +0100
> > Heiko Carstens <hca@...ux.ibm.com> wrote:
> >   
> > > kvm_s390_update_topology_change_report() modifies a single bit within
> > > sca_utility using cmpxchg(). Given that the size of the sca_utility union
> > > is two bytes this generates very inefficient code. Change the size to four
> > > bytes, so better code can be generated.
> > > 
> > > Even though the size of sca_utility doesn't reflect architecture anymore
> > > this seems to be the easiest and most pragmatic approach to avoid
> > > inefficient code.  
> > 
> > wouldn't an atomic bit_op be better in that case?  
> 
> I had that, but decided against it, since the generated code isn't shorter.
> And it would require and unsigned long type within the union, or a cast,
> which I also both disliked.

fair enough

Acked-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>