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Tsirkin" To: Stefan Hajnoczi Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org, Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= Subject: Re: [PATCH] virtio: avoid cost of -ftrivial-auto-var-init in hot path Message-ID: <20250610123707-mutt-send-email-mst@kernel.org> References: <20250604191843.399309-1-stefanha@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20250604191843.399309-1-stefanha@redhat.com> 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: -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=unavailable 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 On Wed, Jun 04, 2025 at 03:18:43PM -0400, Stefan Hajnoczi wrote: > 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%). I ask however whether it is always not worth it for all users. It does reduce chances of leaking stack info, does it not? Maybe we can start with a tri-state Kconfig knob to select between performance/balanced/paranoid for this and similar variables? > 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 > --- > include/qemu/compiler.h | 12 ++++++++++++ > hw/virtio/virtio.c | 8 ++++---- > 2 files changed, 16 insertions(+), 4 deletions(-) > > diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h > index 496dac5ac1..fabd540b02 100644 > --- a/include/qemu/compiler.h > +++ b/include/qemu/compiler.h > @@ -207,6 +207,18 @@ > # define QEMU_USED > #endif > > +/* > + * Disable -ftrivial-auto-var-init on a local variable. Use this in rare cases > + * when the compiler zeroes a large on-stack variable and this causes a > + * performance bottleneck. Only use it when performance data indicates this is > + * necessary since security risks increase with uninitialized stack variables. > + */ > +#if __has_attribute(uninitialized) > +# define QEMU_UNINITIALIZED __attribute__((uninitialized)) > +#else > +# define QEMU_UNINITIALIZED > +#endif > + > /* > * http://clang.llvm.org/docs/ThreadSafetyAnalysis.html > * > 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