linux-kernel - Re: [PATCH v2 0/3] KVM: s390: Couple of small cmpxchg() optimizations

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Message-ID: <20241126131144.0a076101@p-imbrenda>
Date: Tue, 26 Nov 2024 13:11:44 +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 v2 0/3] KVM: s390: Couple of small cmpxchg()
 optimizations

On Tue, 26 Nov 2024 11:25:12 +0100
Heiko Carstens <hca@...ux.ibm.com> wrote:

looks good, and it's a follow-up of your cmpxchg series, so I think it
should go through the s390 kernel tree

(but please try to add the comment in patch 3)

> v2:
> - Replace broken WRITE_ONCE(..., 9) with intended WRITE_ONCE(..., 0).
> 
> v1:
> Use try_cmpxchg() instead of cmpxchg() so compilers with flag output
> operand support (gcc 14 and newer) can generate slightly better code.
> 
> Also get rid of two cmpxchg() usages on one/two byte memory areas
> which generates inefficient code.
> 
> bloat-o-meter statistics of the kvm module:
> 
> add/remove: 0/0 grow/shrink: 0/11 up/down: 0/-318 (-318)
> Function                                     old     new   delta
> kvm_s390_handle_wait                         886     880      -6
> kvm_s390_gisa_destroy                        226     220      -6
> kvm_s390_gisa_clear                           96      90      -6
> ipte_unlock                                  380     372      -8
> kvm_s390_gisc_unregister                     270     260     -10
> kvm_s390_gisc_register                       290     280     -10
> gisa_vcpu_kicker                             200     190     -10
> account_mem                                  250     232     -18
> ipte_lock                                    416     368     -48
> kvm_s390_update_topology_change_report       174     122     -52
> kvm_s390_clear_local_irqs                    420     276    -144
> Total: Before=316521, After=316203, chg -0.10%
> 
> Heiko Carstens (3):
>   KVM: s390: Use try_cmpxchg() instead of cmpxchg() loops
>   KVM: s390: Remove one byte cmpxchg() usage
>   KVM: s390: Increase size of union sca_utility to four bytes
> 
>  arch/s390/include/asm/kvm_host.h | 10 +++++-----
>  arch/s390/kvm/gaccess.c          | 16 ++++++++--------
>  arch/s390/kvm/interrupt.c        | 25 ++++++++-----------------
>  arch/s390/kvm/kvm-s390.c         |  4 ++--
>  arch/s390/kvm/pci.c              |  5 ++---
>  5 files changed, 25 insertions(+), 35 deletions(-)
>