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[80.230.85.71]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4922031b21dsm222512285e9.6.2026.06.15.00.58.01 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 15 Jun 2026 00:58:03 -0700 (PDT) Date: Mon, 15 Jun 2026 03:58:00 -0400 From: "Michael S. Tsirkin" To: Richard Henderson Cc: Peter Maydell , Gavin Shan , qemu-arm@nongnu.org, qemu-devel@nongnu.org, peterx@redhat.com, berrange@redhat.com, david@kernel.org, alex@shazbot.org, clg@redhat.com, pbonzini@redhat.com, philmd@mailo.com, phrdina@redhat.com, jugraham@redhat.com, shan.gavin@gmail.com Subject: Re: [PATCH 1/2] system/memory: Use __builtin_mem{cpy, move} in accessors of ram device region Message-ID: <20260615032545-mutt-send-email-mst@kernel.org> References: <20260612110307.1264798-1-gshan@redhat.com> <20260612110307.1264798-2-gshan@redhat.com> <223f5f94-b9b7-46b0-8466-5be655203a0e@linaro.org> <123d9e80-d731-42c7-81cc-71de47bb13d3@linaro.org> <20260614084004-mutt-send-email-mst@kernel.org> <02d809b0-6396-40dd-a2c5-866028b7cc4d@linaro.org> <20260614131704-mutt-send-email-mst@kernel.org> <52c2af6c-f494-4e30-bac6-bde2aaf48931@linaro.org> MIME-Version: 1.0 In-Reply-To: <52c2af6c-f494-4e30-bac6-bde2aaf48931@linaro.org> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: B5C34erott1VexhjAZSgF_CxItw7lywHqOXZAKWWNoo_1781510285 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=170.10.129.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.445, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-arm@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-arm-bounces+qemu-arm=archiver.kernel.org@nongnu.org Sender: qemu-arm-bounces+qemu-arm=archiver.kernel.org@nongnu.org On Sun, Jun 14, 2026 at 06:41:57PM -0700, Richard Henderson wrote: > On 6/14/26 10:22, Michael S. Tsirkin wrote: > > On Sun, Jun 14, 2026 at 09:45:51AM -0700, Richard Henderson wrote: > > > On 6/14/26 08:13, Michael S. Tsirkin wrote: > > > > Yes, I think it does work because we use -fno-strict-aliasing. > > > > For bigger sizes we'll need packed because the addresses > > > > could be unaligned. > > > ... > > > > For most host/guest pairs things simply work even for unaligned. > > > > > > > > And yes, guest drivers do do this. > > > > > > > > On classical pci, there are no transactions as such and > > > > an unaligned access will be split anyway. > > > > > > I'm saying, if you're talking about pass-through to real devices, that won't > > > work. For instance, AArch64 will trap unaligned accesses to Device memory. > > > > Presumably, AArch64 drivers don't do unaligned at all then? > > Yes. > > > > You need to actually handle unaligned. Perhaps something like > > > > > > /* Find unit to fit size and alignment of dst */ > > > uintptr_t test = (uintptr_t)dst | size; > > > uintptr_t lsb = test & -test; > > > > > > switch (lsb) { > > > case 1: // loop over uint8_t > > > case 2: // loop over uint16_t > > > case 4: // loop over uint32_t > > > default: // loop over uint64_t > > > } > > > > > > with the expectation that normally we'll have aligned addresses and size > > > such that the loop will iterate once. > > > > > > > > > r~ > > > > And ifdef for arches without unaligned support? > > No ifdef. All accesses produced by the above are aligned. > > > r~ So something like: #if defined(__i386__) || defined(__x86_64__) #define HOST_UNALIGNED_MMIO_OK 1 #else #define HOST_UNALIGNED_MMIO_OK 0 #endif and then we can check that. -- MST