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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 02/31] hw/virtio/virtio: avoid cost of -ftrivial-auto-var-init in hot path Date: Tue, 10 Jun 2025 13:36:40 +0100 Message-ID: <20250610123709.835102-3-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: 8bit X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass client-ip=170.10.133.124; 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+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org From: Stefan Hajnoczi Since commit 7ff9ff039380 ("meson: mitigate against use of uninitialize stack for exploits") the -ftrivial-auto-var-init=zero compiler option is used to zero local variables. While this reduces security risks associated with uninitialized stack data, it introduced a measurable bottleneck in the virtqueue_split_pop() and virtqueue_packed_pop() functions. These virtqueue functions are in the hot path. They are called for each element (request) that is popped from a VIRTIO device's virtqueue. Using __attribute__((uninitialized)) on large stack variables in these functions improves fio randread bs=4k iodepth=64 performance from 304k to 332k IOPS (+9%). This issue was found using perf-top(1). virtqueue_split_pop() was one of the top CPU consumers and the "annotate" feature showed that the memory zeroing instructions at the beginning of the functions were hot. Fixes: 7ff9ff039380 ("meson: mitigate against use of uninitialize stack for exploits") Cc: Daniel P. Berrangé Signed-off-by: Stefan Hajnoczi --- hw/virtio/virtio.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c index 5534251e01..82a285a31d 100644 --- a/hw/virtio/virtio.c +++ b/hw/virtio/virtio.c @@ -1689,8 +1689,8 @@ static void *virtqueue_split_pop(VirtQueue *vq, size_t sz) VirtIODevice *vdev = vq->vdev; VirtQueueElement *elem = NULL; unsigned out_num, in_num, elem_entries; - hwaddr addr[VIRTQUEUE_MAX_SIZE]; - struct iovec iov[VIRTQUEUE_MAX_SIZE]; + hwaddr QEMU_UNINITIALIZED addr[VIRTQUEUE_MAX_SIZE]; + struct iovec QEMU_UNINITIALIZED iov[VIRTQUEUE_MAX_SIZE]; VRingDesc desc; int rc; @@ -1836,8 +1836,8 @@ static void *virtqueue_packed_pop(VirtQueue *vq, size_t sz) VirtIODevice *vdev = vq->vdev; VirtQueueElement *elem = NULL; unsigned out_num, in_num, elem_entries; - hwaddr addr[VIRTQUEUE_MAX_SIZE]; - struct iovec iov[VIRTQUEUE_MAX_SIZE]; + hwaddr QEMU_UNINITIALIZED addr[VIRTQUEUE_MAX_SIZE]; + struct iovec QEMU_UNINITIALIZED iov[VIRTQUEUE_MAX_SIZE]; VRingPackedDesc desc; uint16_t id; int rc; -- 2.49.0