66 lines
2.6 KiB
Diff
66 lines
2.6 KiB
Diff
From ba2868ce091cd4abe4be6de4b7e44b3be303b352 Mon Sep 17 00:00:00 2001
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From: Stefan Hajnoczi <stefanha@redhat.com>
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Date: Tue, 10 Jun 2025 13:36:40 +0100
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Subject: [PATCH] hw/virtio/virtio: avoid cost of -ftrivial-auto-var-init in
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hot path
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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Since commit 7ff9ff039380 ("meson: mitigate against use of uninitialize
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stack for exploits") the -ftrivial-auto-var-init=zero compiler option is
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used to zero local variables. While this reduces security risks
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associated with uninitialized stack data, it introduced a measurable
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bottleneck in the virtqueue_split_pop() and virtqueue_packed_pop()
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functions.
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These virtqueue functions are in the hot path. They are called for each
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element (request) that is popped from a VIRTIO device's virtqueue. Using
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__attribute__((uninitialized)) on large stack variables in these
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functions improves fio randread bs=4k iodepth=64 performance from 304k
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to 332k IOPS (+9%).
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This issue was found using perf-top(1). virtqueue_split_pop() was one of
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the top CPU consumers and the "annotate" feature showed that the memory
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zeroing instructions at the beginning of the functions were hot.
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Fixes: 7ff9ff039380 ("meson: mitigate against use of uninitialize stack for exploits")
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Cc: Daniel P. Berrangé <berrange@redhat.com>
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Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
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Message-id: 20250610123709.835102-3-berrange@redhat.com
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Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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---
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hw/virtio/virtio.c | 8 ++++----
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1 file changed, 4 insertions(+), 4 deletions(-)
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diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
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index 5534251e01..82a285a31d 100644
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--- a/hw/virtio/virtio.c
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+++ b/hw/virtio/virtio.c
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@@ -1689,8 +1689,8 @@ static void *virtqueue_split_pop(VirtQueue *vq, size_t sz)
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VirtIODevice *vdev = vq->vdev;
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VirtQueueElement *elem = NULL;
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unsigned out_num, in_num, elem_entries;
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- hwaddr addr[VIRTQUEUE_MAX_SIZE];
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- struct iovec iov[VIRTQUEUE_MAX_SIZE];
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+ hwaddr QEMU_UNINITIALIZED addr[VIRTQUEUE_MAX_SIZE];
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+ struct iovec QEMU_UNINITIALIZED iov[VIRTQUEUE_MAX_SIZE];
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VRingDesc desc;
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int rc;
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@@ -1836,8 +1836,8 @@ static void *virtqueue_packed_pop(VirtQueue *vq, size_t sz)
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VirtIODevice *vdev = vq->vdev;
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VirtQueueElement *elem = NULL;
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unsigned out_num, in_num, elem_entries;
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- hwaddr addr[VIRTQUEUE_MAX_SIZE];
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- struct iovec iov[VIRTQUEUE_MAX_SIZE];
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+ hwaddr QEMU_UNINITIALIZED addr[VIRTQUEUE_MAX_SIZE];
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+ struct iovec QEMU_UNINITIALIZED iov[VIRTQUEUE_MAX_SIZE];
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VRingPackedDesc desc;
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uint16_t id;
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int rc;
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--
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2.47.3
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