Re: [PATCH 1/2] Reduce vdpa initialization / startup overhead

[Jason Wang]

On Wed, Apr 19, 2023 at 11:33 PM Eugenio Perez Martin
<eperezma@redhat.com> wrote:
>
> On Wed, Apr 19, 2023 at 12:56 AM <peili.dev@gmail.com> wrote:
> >
> > From: Pei Li <peili.dev@gmail.com>
> >
> > Currently, part of the vdpa initialization / startup process
> > needs to trigger many ioctls per vq, which is very inefficient
> > and causing unnecessary context switch between user mode and
> > kernel mode.
> >
> > This patch creates an additional ioctl() command, namely
> > VHOST_VDPA_GET_VRING_GROUP_BATCH, that will batching
> > commands of VHOST_VDPA_GET_VRING_GROUP into a single
> > ioctl() call.

I'd expect there's a kernel patch but I didn't see that?

If we want to go this way. Why simply have a more generic way, that is
introducing something like:

VHOST_CMD_BATCH which did something like

struct vhost_cmd_batch {
    int ncmds;
    struct vhost_ioctls[];
};

Then you can batch other ioctls other than GET_VRING_GROUP?

Thanks

> >
>
> It seems to me you forgot to send the 0/2 cover letter :).
>
> Please include the time we save thanks to avoiding the repetitive
> ioctls in each patch.
>
> CCing Jason and Michael.
>
> > Signed-off-by: Pei Li <peili.dev@gmail.com>
> > ---
> >  hw/virtio/vhost-vdpa.c                       | 31 +++++++++++++++-----
> >  include/standard-headers/linux/vhost_types.h |  3 ++
> >  linux-headers/linux/vhost.h                  |  7 +++++
> >  3 files changed, 33 insertions(+), 8 deletions(-)
> >
> > diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
> > index bc6bad23d5..6d45ff8539 100644
> > --- a/hw/virtio/vhost-vdpa.c
> > +++ b/hw/virtio/vhost-vdpa.c
> > @@ -679,7 +679,8 @@ static int vhost_vdpa_set_backend_cap(struct vhost_dev 
> > *dev)
> >      uint64_t f = 0x1ULL << VHOST_BACKEND_F_IOTLB_MSG_V2 |
> >          0x1ULL << VHOST_BACKEND_F_IOTLB_BATCH |
> >          0x1ULL << VHOST_BACKEND_F_IOTLB_ASID |
> > -        0x1ULL << VHOST_BACKEND_F_SUSPEND;
> > +        0x1ULL << VHOST_BACKEND_F_SUSPEND |
> > +        0x1ULL << VHOST_BACKEND_F_IOCTL_BATCH;
> >      int r;
> >
> >      if (vhost_vdpa_call(dev, VHOST_GET_BACKEND_FEATURES, &features)) {
> > @@ -731,14 +732,28 @@ static int vhost_vdpa_get_vq_index(struct vhost_dev 
> > *dev, int idx)
> >
> >  static int vhost_vdpa_set_vring_ready(struct vhost_dev *dev)
> >  {
> > -    int i;
> > +    int i, nvqs = dev->nvqs;
> > +    uint64_t backend_features = dev->backend_cap;
> > +
> >      trace_vhost_vdpa_set_vring_ready(dev);
> > -    for (i = 0; i < dev->nvqs; ++i) {
> > -        struct vhost_vring_state state = {
> > -            .index = dev->vq_index + i,
> > -            .num = 1,
> > -        };
> > -        vhost_vdpa_call(dev, VHOST_VDPA_SET_VRING_ENABLE, &state);
> > +
> > +    if (!(backend_features & BIT_ULL(VHOST_BACKEND_F_IOCTL_BATCH))) {
> > +        for (i = 0; i < nvqs; ++i) {
> > +            struct vhost_vring_state state = {
> > +                .index = dev->vq_index + i,
> > +                .num = 1,
> > +            };
> > +            vhost_vdpa_call(dev, VHOST_VDPA_SET_VRING_ENABLE, &state);
> > +        }
> > +    } else {
> > +        struct vhost_vring_state states[nvqs + 1];
> > +        states[0].num = nvqs;
> > +        for (i = 1; i <= nvqs; ++i) {
> > +            states[i].index = dev->vq_index + i - 1;
> > +            states[i].num = 1;
> > +        }
> > +
>
> I think it's more clear to share the array of vhost_vring_state
> states[nvqs + 1], and then fill it either in one shot with
> VHOST_VDPA_SET_VRING_ENABLE_BATCH or individually with
> VHOST_VDPA_SET_VRING_ENABLE.
>
> The same feedback goes for VHOST_VDPA_GET_VRING_GROUP_BATCH in the next patch.
>
> > +        vhost_vdpa_call(dev, VHOST_VDPA_SET_VRING_ENABLE_BATCH, 
> > &states[0]);
> >      }
> >      return 0;
>
> This comment is previous to your patch but I just realized we don't
> check the return value of vhost_vdpa_call. Maybe it is worth
> considering addressing that in this series too.
>
> >  }
> > diff --git a/include/standard-headers/linux/vhost_types.h 
> > b/include/standard-headers/linux/vhost_types.h
> > index c41a73fe36..068d0e1ceb 100644
> > --- a/include/standard-headers/linux/vhost_types.h
> > +++ b/include/standard-headers/linux/vhost_types.h
> > @@ -164,4 +164,7 @@ struct vhost_vdpa_iova_range {
> >  /* Device can be suspended */
> >  #define VHOST_BACKEND_F_SUSPEND  0x4
> >
> > +/* IOCTL requests can be batched */
> > +#define VHOST_BACKEND_F_IOCTL_BATCH 0x6
> > +
> >  #endif
> > diff --git a/linux-headers/linux/vhost.h b/linux-headers/linux/vhost.h
> > index f9f115a7c7..4c9ddd0a0e 100644
> > --- a/linux-headers/linux/vhost.h
> > +++ b/linux-headers/linux/vhost.h
> > @@ -180,4 +180,11 @@
> >   */
> >  #define VHOST_VDPA_SUSPEND             _IO(VHOST_VIRTIO, 0x7D)
> >
> > +/* Batch version of VHOST_VDPA_SET_VRING_ENABLE
> > + *
> > + * Enable/disable the ring while batching the commands.
> > + */
> > +#define VHOST_VDPA_SET_VRING_ENABLE_BATCH      _IOW(VHOST_VIRTIO, 0x7F, \
> > +                                            struct vhost_vring_state)
> > +
>
> These headers should be updated with update-linux-headers.sh. To be
> done that way we need the changes merged in the kernel before.
>
> We can signal that the series is not ready for inclusion with the
> subject [RFC. PATCH], like [1], and note it in the commit message.
> That way, you can keep updating the header manually for demonstration
> purposes.
>
> Thanks!
>
> [1] 
> https://lore.kernel.org/qemu-devel/20220413163206.1958254-23-eperezma@redhat.com/
>