Re: [Qemu-devel] [PATCH] virtio: add support for in-order feature

[Michael S. Tsirkin]

On Fri, Aug 10, 2018 at 11:28:47AM +0300, Ilya Maximets wrote:
> On 10.08.2018 01:51, Michael S. Tsirkin wrote:
> > On Thu, Aug 09, 2018 at 07:54:37PM +0300, Ilya Maximets wrote:
> >> New feature bit for in-order feature of the upcoming
> >> virtio 1.1. It's already supported by DPDK vhost-user
> >> and virtio implementations. These changes required to
> >> allow feature negotiation.
> >>
> >> Signed-off-by: Ilya Maximets <address@hidden>
> >> ---
> >>
> >> I just wanted to test this new feature in DPDK but failed
> >> to found required patch for QEMU side. So, I implemented it.
> >> At least it will be helpful for someone like me, who wants
> >> to evaluate VIRTIO_F_IN_ORDER with DPDK.
> >>
> >>  hw/net/vhost_net.c                             |  1 +
> >>  include/hw/virtio/virtio.h                     | 12 +++++++-----
> >>  include/standard-headers/linux/virtio_config.h |  7 +++++++
> >>  3 files changed, 15 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
> >> index e037db6..86879c5 100644
> >> --- a/hw/net/vhost_net.c
> >> +++ b/hw/net/vhost_net.c
> >> @@ -78,6 +78,7 @@ static const int user_feature_bits[] = {
> >>      VIRTIO_NET_F_MRG_RXBUF,
> >>      VIRTIO_NET_F_MTU,
> >>      VIRTIO_F_IOMMU_PLATFORM,
> >> +    VIRTIO_F_IN_ORDER,
> >>  
> >>      /* This bit implies RARP isn't sent by QEMU out of band */
> >>      VIRTIO_NET_F_GUEST_ANNOUNCE,
> >> diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> >> index 9c1fa07..a422025 100644
> >> --- a/include/hw/virtio/virtio.h
> >> +++ b/include/hw/virtio/virtio.h
> >> @@ -254,16 +254,18 @@ typedef struct virtio_input_conf virtio_input_conf;
> >>  typedef struct VirtIOSCSIConf VirtIOSCSIConf;
> >>  typedef struct VirtIORNGConf VirtIORNGConf;
> >>  
> >> -#define DEFINE_VIRTIO_COMMON_FEATURES(_state, _field) \
> >> +#define DEFINE_VIRTIO_COMMON_FEATURES(_state, _field)     \
> >>      DEFINE_PROP_BIT64("indirect_desc", _state, _field,    \
> >>                        VIRTIO_RING_F_INDIRECT_DESC, true), \
> >>      DEFINE_PROP_BIT64("event_idx", _state, _field,        \
> >>                        VIRTIO_RING_F_EVENT_IDX, true),     \
> >>      DEFINE_PROP_BIT64("notify_on_empty", _state, _field,  \
> >> -                      VIRTIO_F_NOTIFY_ON_EMPTY, true), \
> >> -    DEFINE_PROP_BIT64("any_layout", _state, _field, \
> >> -                      VIRTIO_F_ANY_LAYOUT, true), \
> >> -    DEFINE_PROP_BIT64("iommu_platform", _state, _field, \
> >> +                      VIRTIO_F_NOTIFY_ON_EMPTY, true),    \
> >> +    DEFINE_PROP_BIT64("any_layout", _state, _field,       \
> >> +                      VIRTIO_F_ANY_LAYOUT, true),         \
> >> +    DEFINE_PROP_BIT64("in_order", _state, _field,         \
> >> +                      VIRTIO_F_IN_ORDER, true),           \
> >> +    DEFINE_PROP_BIT64("iommu_platform", _state, _field,   \
> >>                        VIRTIO_F_IOMMU_PLATFORM, false)
> > 
> > Is in_order really right for all virtio devices?
> 
> I see nothing device specific in this feature. It just specifies
> some restrictions on the descriptors handling. All virtio devices
> could use it to have performance benefits. Also, upcoming packed
> rings should give a good performance boost in case of enabled
> in-order feature. And packed rings RFC [1] implements
> VIRTIO_F_RING_PACKED for all virtio devices. So, I see no issues
> in enabling in-order negotiation for all of them.
> 
> What do you think?
> 
> [1] https://lists.gnu.org/archive/html/qemu-devel/2018-06/msg01028.html
> 
> Best regards, Ilya Maximets.

If guest assumes in-order use of buffers but device uses them out of
order then guest will crash. So there's a missing piece where
you actually make devices use buffers in order when the flag is set.

> > 
> >>  hwaddr virtio_queue_get_desc_addr(VirtIODevice *vdev, int n);
> >> diff --git a/include/standard-headers/linux/virtio_config.h 
> >> b/include/standard-headers/linux/virtio_config.h
> >> index b777069..d20398c 100644
> >> --- a/include/standard-headers/linux/virtio_config.h
> >> +++ b/include/standard-headers/linux/virtio_config.h
> >> @@ -71,4 +71,11 @@
> >>   * this is for compatibility with legacy systems.
> >>   */
> >>  #define VIRTIO_F_IOMMU_PLATFORM           33
> >> +
> >> +/*
> >> + * Inorder feature indicates that all buffers are used by the device
> >> + * in the same order in which they have been made available.
> >> + */
> >> +#define VIRTIO_F_IN_ORDER 35
> >> +
> >>  #endif /* _LINUX_VIRTIO_CONFIG_H */
> >> -- 
> >> 2.7.4
> > 
> >