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Tsirkin" , Peter Maydell , qemu-block@nongnu.org, Stefan Hajnoczi , Kevin Wolf , =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= Subject: [PATCH 23/31] hw/nvme/ctrl: skip automatic zero-init of large arrays Date: Tue, 10 Jun 2025 13:37:01 +0100 Message-ID: <20250610123709.835102-24-berrange@redhat.com> In-Reply-To: <20250610123709.835102-1-berrange@redhat.com> References: <20250610123709.835102-1-berrange@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, 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_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=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: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1749559832744116600 The 'nvme_map_sgl' method has a 256 element array used for copying data from the device. Skip the automatic zero-init of this array to eliminate the performance overhead in the I/O hot path. The 'segment' array will be fully initialized when reading data from the device. The 'nme_changed_nslist' method has a 4k byte array that is manually initialized with memset(). The compiler ought to be intelligent enough to turn the memset() into a static initialization operation, and thus not duplicate the automatic zero-init. Replacing memset() with '{}' makes it unambiguous that the array is statically initialized. Signed-off-by: Daniel P. Berrang=C3=A9 Reviewed-by: Klaus Jensen --- hw/nvme/ctrl.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/hw/nvme/ctrl.c b/hw/nvme/ctrl.c index fd935507bc..220002830d 100644 --- a/hw/nvme/ctrl.c +++ b/hw/nvme/ctrl.c @@ -1057,7 +1057,8 @@ static uint16_t nvme_map_sgl(NvmeCtrl *n, NvmeSg *sg,= NvmeSglDescriptor sgl, */ #define SEG_CHUNK_SIZE 256 =20 - NvmeSglDescriptor segment[SEG_CHUNK_SIZE], *sgld, *last_sgld; + QEMU_UNINITIALIZED NvmeSglDescriptor segment[SEG_CHUNK_SIZE]; + NvmeSglDescriptor *sgld, *last_sgld; uint64_t nsgld; uint32_t seg_len; uint16_t status; @@ -5128,7 +5129,7 @@ static uint16_t nvme_error_info(NvmeCtrl *n, uint8_t = rae, uint32_t buf_len, static uint16_t nvme_changed_nslist(NvmeCtrl *n, uint8_t rae, uint32_t buf= _len, uint64_t off, NvmeRequest *req) { - uint32_t nslist[1024]; + uint32_t nslist[1024] =3D {}; uint32_t trans_len; int i =3D 0; uint32_t nsid; @@ -5138,7 +5139,6 @@ static uint16_t nvme_changed_nslist(NvmeCtrl *n, uint= 8_t rae, uint32_t buf_len, return NVME_INVALID_FIELD | NVME_DNR; } =20 - memset(nslist, 0x0, sizeof(nslist)); trans_len =3D MIN(sizeof(nslist) - off, buf_len); =20 while ((nsid =3D find_first_bit(n->changed_nsids, NVME_CHANGED_NSID_SI= ZE)) !=3D --=20 2.49.0