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

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Aug 10, 2018 at 02:04:47PM +0300, Ilya Maximets wrote:
> On 10.08.2018 12:34, Michael S. Tsirkin wrote:
> > 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 <i.maximets@samsung.com>
> >>>> ---
> >>>>
> >>>> 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.
> 
> I thought that feature negotiation is the mechanism that should
> protect us from situations like that. Isn't it?
> If device negotiates in-order feature, when it MUST (as described
> in spec) use buffers in the same order in which they have been
> available.

Exactly. And your patch does nothing to ensure that,
or limit to devices which use buffers in order.

> > 
> >>>
> >>>>  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
> >>>
> >>>
> > 
> >