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[31.46.241.165]) by smtp.gmail.com with ESMTPSA id 12-20020a0564021f4c00b0056e62c8e3bcsm1993054edz.54.2024.04.09.00.50.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 09 Apr 2024 00:50:52 -0700 (PDT) Sender: Ingo Molnar Date: Tue, 9 Apr 2024 09:50:50 +0200 From: Ingo Molnar To: Uros Bizjak Cc: x86@kernel.org, linux-kernel@vger.kernel.org, Thomas Gleixner , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , Peter Zijlstra Subject: Re: [PATCH 3/3] locking/atomic/x86: Introduce arch_try_cmpxchg64() for !CONFIG_X86_CMPXCHG64 Message-ID: References: <20240408091547.90111-1-ubizjak@gmail.com> <20240408091547.90111-4-ubizjak@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240408091547.90111-4-ubizjak@gmail.com> * Uros Bizjak wrote: > Commit: > > 6d12c8d308e68 ("percpu: Wire up cmpxchg128") > > improved emulated cmpxchg8b_emu() library function to return > success/failure in a ZF flag. > > Define arch_try_cmpxchg64() for !CONFIG_X86_CMPXCHG64 targets > to override the generic archy_try_cmpxchg() with an optimized > target specific implementation that handles ZF flag. > > The assembly code at the call sites improves from: > > bf56d: e8 fc ff ff ff call cmpxchg8b_emu > bf572: 8b 74 24 28 mov 0x28(%esp),%esi > bf576: 89 c3 mov %eax,%ebx > bf578: 89 d1 mov %edx,%ecx > bf57a: 8b 7c 24 2c mov 0x2c(%esp),%edi > bf57e: 89 f0 mov %esi,%eax > bf580: 89 fa mov %edi,%edx > bf582: 31 d8 xor %ebx,%eax > bf584: 31 ca xor %ecx,%edx > bf586: 09 d0 or %edx,%eax > bf588: 0f 84 e3 01 00 00 je bf771 <...> > > to: > > bf572: e8 fc ff ff ff call cmpxchg8b_emu > bf577: 0f 84 b6 01 00 00 je bf733 <...> > > No functional changes intended. Side note: while there's no hard-written rule for it, I tend to use the 'no functional changes intended' line for pure identity transformations - which this one isn't, as it changes code generation materially. So I removed that line - the explanation of the patch is clear enough IMO. Thanks, Ingo