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The virtio specification is not completely clear about seg_max and its relationship with queue_size. Some drivers (for example, the modern Linux kernel driver) rely on seg_max to set the maximum number of segments used in a request. If seg_max is set larger than queue_size, such a driver might overwhelm a virtqueue by trying to send more segments than it can handle. As a result, it either hangs or faults. One might argue that it is the vhost-user server's responsibility to set a valid seg_max value. However, due to the issue described in the previous paragraph, this value should generally depend on queue_size. That's why it may be necessary to control it on QEMU's side. This patch adds the seg-max-adjust flag. A flag with the same name already exists for virtio-blk (and it exists to solve the same problem, except that here we have a vhost-user server to consult). If seg-max-adjust is set, the final seg_max is the minimum of the value provided by the vhost-user server and (queue_size - 2). It is not enabled by default. Signed-off-by: Sergei Heifetz <heifetz@yandex-team.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Message-ID: <20260401230548.136541-1-heifetz@yandex-team.com>