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[83.51.215.31]) by smtp.gmail.com with ESMTPSA id jo12sm5074052ejb.22.2021.05.18.22.56.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 18 May 2021 22:56:53 -0700 (PDT) Subject: Re: [RFC PATCH 14/25] qemu/bswap: Introduce load/store for aligned pointer To: Peter Maydell , Stefan Hajnoczi , Paolo Bonzini References: <20210518183655.1711377-1-philmd@redhat.com> <20210518183655.1711377-15-philmd@redhat.com> From: =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= Message-ID: <82c66450-24de-89a2-71fb-79dcdebd1baa@redhat.com> Date: Wed, 19 May 2021 07:56:51 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=philmd@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=philmd@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -31 X-Spam_score: -3.2 X-Spam_bar: --- X-Spam_report: (-3.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.374, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Emanuele Giuseppe Esposito , "Michael S. Tsirkin" , Huacai Chen , Richard Henderson , QEMU Developers , Peter Xu , Bibo Mao Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 5/18/21 10:15 PM, Peter Maydell wrote: > On Tue, 18 May 2021 at 19:38, Philippe Mathieu-Daudé wrote: >> >> When the pointer alignment is known to be safe, we can >> directly swap the data in place, without having to rely >> on the compiler builtin code. >> >> Load/store methods expecting aligned pointer use the 'a' >> infix. For example to read a 16-bit unsigned value stored >> in little endianess at an unaligned pointer: >> >> val = lduw_le_p(&unaligned_ptr); >> >> then to store it in big endianess at an aligned pointer: >> >> stw_be_ap(&aligned_ptr, val); > > It sounded from the bug report as if the desired effect > was "this access is atomic". Nothing in the documentation here > makes that guarantee of the implementation -- it merely imposes > an extra requirement on the caller that the pointer alignment > is "safe" (which term it does not define...) and a valid > implementation would be to implement the "aligned" versions > identically to the "unaligned" versions... > > Q: should the functions at the bottom of this stack of APIs > be using something from the atomic.h header? If not, why not? > Do we need any of the other atomic primitives ? I'll defer this question to Stefan/Paolo...