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[80.230.85.71]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-490ea961f18sm281397045e9.2.2026.06.15.09.37.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 15 Jun 2026 09:37:53 -0700 (PDT) Date: Mon, 15 Jun 2026 12:37:49 -0400 From: "Michael S. Tsirkin" To: Richard Henderson Cc: Gavin Shan , qemu-arm@nongnu.org, qemu-devel@nongnu.org, peterx@redhat.com, alex@shazbot.org, peter.maydell@linaro.org, berrange@redhat.com, philmd@oss.qualcomm.com, philmd@mailo.com, david@kernel.org, clg@redhat.com, pbonzini@redhat.com, phrdina@redhat.com, jugraham@redhat.com, liugang24219@sangfor.com.cn, dinghui@sangfor.com.cn, shan.gavin@gmail.com Subject: Re: [PATCH v2 1/2] system/memory: Use qemu_ram_{copy, move}() in ram device region accessors Message-ID: <20260615123633-mutt-send-email-mst@kernel.org> References: <20260615100200.266968-1-gshan@redhat.com> <20260615100200.266968-2-gshan@redhat.com> <93fb3b41-1543-4e3b-922b-043b774f57bf@linaro.org> <20260615123354-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260615123354-mutt-send-email-mst@kernel.org> 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-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Mon, Jun 15, 2026 at 12:36:00PM -0400, Michael S. Tsirkin wrote: > On Mon, Jun 15, 2026 at 08:17:14AM -0700, Richard Henderson wrote: > > On 6/15/26 03:01, Gavin Shan wrote: > > > +void qemu_ram_copy(void *dest, const void *src, size_t n) > > > +{ > > > + if (HOST_UNALIGNED_MMIO_OK) { > > > + switch (n) { > > > + case 1: > > > + __builtin_memcpy(dest, src, 1); > > > + break; > > > + case 2: > > > + __builtin_memcpy(dest, src, 2); > > > + break; > > > + case 4: > > > + __builtin_memcpy(dest, src, 4); > > > + break; > > > + case 8: > > > + __builtin_memcpy(dest, src, 8); > > > + break; > > > + default: > > > + memcpy(dest, src, n); > > > + } > > > + } else { > > > + uintptr_t test, lsb; > > > + > > > + do { > > > + test = (uintptr_t)dest | n; > > > + lsb = test & -test; > > > + switch (lsb) { > > > > Either assert n != 0 to start, or use while not do/while. > > Because the body of the loop won't handle n == 0 correctly. I think n!=0 performs better than a loop. Worth checking asm though. > > > + case 1: > > > + *(uint8_t *)dest = *(uint8_t *)src; > > > + src += 1; > > > + dest += 1; > > > + n -= 1; > > > + break; > > > + case 2: > > > + *(uint16_t *)dest = *(uint16_t *)src; > > > + src += 2; > > > + dest += 2; > > > + n -= 2; > > > + break; > > > + case 4: > > > + *(uint32_t *)dest = *(uint32_t *)src; > > > + src += 4; > > > + dest += 4; > > > + n -= 4; > > > + break; > > > + default: > > > + *(uint64_t *)dest = *(uint64_t *)src; > > > + src += 8; > > > + dest += 8; > > > + n -= 8; > > > > Use qatomic_set for the stores. > > > Won't this do things like locked on x86? Hmm I didn't check. It's relaxed - so no locked. Fine by me. > > > > > > src is not aligned, so except for case 1, you need ld{uw,l,q}_he_p. > > These just map back to memcpy. What is the point? > > > > +void qemu_ram_move(void *dest, const void *src, size_t n) > > > +{ > > > + if (HOST_UNALIGNED_MMIO_OK) { > > > + switch (n) { > > > + case 1: > > > + __builtin_memmove(dest, src, 1); > > > + break; > > > + case 2: > > > + __builtin_memmove(dest, src, 2); > > > + break; > > > + case 4: > > > + __builtin_memmove(dest, src, 4); > > > + break; > > > + case 8: > > > + __builtin_memmove(dest, src, 8); > > > + break; > > > + default: > > > + memmove(dest, src, n); > > > + } > > > + } else { > > > + qemu_ram_copy(dest, src, n); > > > + } > > > +} > > > > The qemu_ram_copy implementation above does not work with overlapping blocks. > > IIUC copy is not supposed to. this is what move is for. > > > > > r~