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Date: Fri, 30 Mar 2018 10:05:18 +0800
From: Tiwei Bie <tiwei.bie@...el.com>
To: Jason Wang <jasowang@...hat.com>
Cc: mst@...hat.com, virtualization@...ts.linux-foundation.org,
kvm@...r.kernel.org, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH V2 8/8] vhost: event suppression for packed ring
On Mon, Mar 26, 2018 at 11:38:53AM +0800, Jason Wang wrote:
> This patch introduces basic support for event suppression aka driver
> and device area. Compile tested only.
>
> Signed-off-by: Jason Wang <jasowang@...hat.com>
> ---
[...]
> +
> +static bool vhost_notify_packed(struct vhost_dev *dev,
> + struct vhost_virtqueue *vq)
> +{
> + __virtio16 event_off_wrap, event_flags;
> + __u16 old, new;
> + bool v, wrap;
> + int off;
> +
> + /* Flush out used descriptors updates. This is paired
> + * with the barrier that the Guest executes when enabling
> + * interrupts.
> + */
> + smp_mb();
> +
> + if (vhost_get_avail(vq, event_flags,
> + &vq->driver_event->desc_event_flags) < 0) {
> + vq_err(vq, "Failed to get driver desc_event_flags");
> + return true;
> + }
> +
> + if (!(event_flags & cpu_to_vhost16(vq, RING_EVENT_FLAGS_DESC)))
> + return event_flags ==
> + cpu_to_vhost16(vq, RING_EVENT_FLAGS_ENABLE);
Maybe it would be better to not use '&' here. Because these flags
are not defined as bits which can be ORed or ANDed. Instead, they
are defined as values:
0x0 enable
0x1 disable
0x2 desc
0x3 reserved
> +
> + /* Read desc event flags before event_off and event_wrap */
> + smp_rmb();
> +
> + if (vhost_get_avail(vq, event_off_wrap,
> + &vq->driver_event->desc_event_off_warp) < 0) {
> + vq_err(vq, "Failed to get driver desc_event_off/wrap");
> + return true;
> + }
> +
> + off = vhost16_to_cpu(vq, event_off_wrap);
> +
> + wrap = off & 0x1;
> + off >>= 1;
Based on the below definitions in spec, wrap counter is
the most significant bit.
struct pvirtq_event_suppress {
le16 {
desc_event_off : 15; /* Descriptor Ring Change Event Offset */
desc_event_wrap : 1; /* Descriptor Ring Change Event Wrap Counter */
} desc; /* If desc_event_flags set to RING_EVENT_FLAGS_DESC */
le16 {
desc_event_flags : 2, /* Descriptor Ring Change Event Flags */
reserved : 14; /* Reserved, set to 0 */
} flags;
};
> +
> +
> + old = vq->signalled_used;
> + v = vq->signalled_used_valid;
> + new = vq->signalled_used = vq->last_used_idx;
> + vq->signalled_used_valid = true;
> +
> + if (unlikely(!v))
> + return true;
> +
> + return vhost_vring_packed_need_event(vq, new, old, off) &&
> + wrap == vq->used_wrap_counter;
> +}
> +
> +static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
> +{
> + if (vhost_has_feature(vq, VIRTIO_F_RING_PACKED))
> + return vhost_notify_packed(dev, vq);
> + else
> + return vhost_notify_split(dev, vq);
> +}
> +
> /* This actually signals the guest, using eventfd. */
> void vhost_signal(struct vhost_dev *dev, struct vhost_virtqueue *vq)
> {
> @@ -2789,7 +2911,17 @@ static bool vhost_enable_notify_packed(struct vhost_dev *dev,
> __virtio16 flags;
> int ret;
>
> - /* FIXME: disable notification through device area */
> + if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
> + return false;
> + vq->used_flags &= ~VRING_USED_F_NO_NOTIFY;
> +
> + flags = cpu_to_vhost16(vq, RING_EVENT_FLAGS_ENABLE);
> + ret = vhost_update_device_flags(vq, flags);
> + if (ret) {
> + vq_err(vq, "Failed to enable notification at %p: %d\n",
> + &vq->device_event->desc_event_flags, ret);
> + return false;
> + }
>
> /* They could have slipped one in as we were doing that: make
> * sure it's written, then check again. */
> @@ -2855,7 +2987,18 @@ EXPORT_SYMBOL_GPL(vhost_enable_notify);
> static void vhost_disable_notify_packed(struct vhost_dev *dev,
> struct vhost_virtqueue *vq)
> {
> - /* FIXME: disable notification through device area */
> + __virtio16 flags;
> + int r;
> +
> + if (vq->used_flags & VRING_USED_F_NO_NOTIFY)
> + return;
> + vq->used_flags |= VRING_USED_F_NO_NOTIFY;
> +
> + flags = cpu_to_vhost16(vq, RING_EVENT_FLAGS_DISABLE);
> + r = vhost_update_device_flags(vq, flags);
> + if (r)
> + vq_err(vq, "Failed to enable notification at %p: %d\n",
> + &vq->device_event->desc_event_flags, r);
> }
>
> static void vhost_disable_notify_split(struct vhost_dev *dev,
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 8a9df4f..02d7a36 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -96,8 +96,14 @@ struct vhost_virtqueue {
> struct vring_desc __user *desc;
> struct vring_desc_packed __user *desc_packed;
Do you think it'd be better to name the desc type as
struct vring_packed_desc? And it will be consistent
with other names, like:
struct vring_packed;
struct vring_packed_desc_event;
> };
> - struct vring_avail __user *avail;
> - struct vring_used __user *used;
> + union {
> + struct vring_avail __user *avail;
> + struct vring_packed_desc_event __user *driver_event;
> + };
> + union {
> + struct vring_used __user *used;
> + struct vring_packed_desc_event __user *device_event;
> + };
> const struct vhost_umem_node *meta_iotlb[VHOST_NUM_ADDRS];
> struct file *kick;
> struct eventfd_ctx *call_ctx;
> diff --git a/include/uapi/linux/virtio_ring.h b/include/uapi/linux/virtio_ring.h
> index e297580..7cdbf06 100644
> --- a/include/uapi/linux/virtio_ring.h
> +++ b/include/uapi/linux/virtio_ring.h
> @@ -75,6 +75,25 @@ struct vring_desc_packed {
> __virtio16 flags;
> };
>
> +/* Enable events */
> +#define RING_EVENT_FLAGS_ENABLE 0x0
> +/* Disable events */
> +#define RING_EVENT_FLAGS_DISABLE 0x1
> +/*
> + * Enable events for a specific descriptor
> + * (as specified by Descriptor Ring Change Event Offset/Wrap Counter).
> + * Only valid if VIRTIO_F_RING_EVENT_IDX has been negotiated.
> + */
> +#define RING_EVENT_FLAGS_DESC 0x2
> +/* The value 0x3 is reserved */
> +
> +struct vring_packed_desc_event {
> + /* Descriptor Ring Change Event Offset and Wrap Counter */
> + __virtio16 desc_event_off_warp;
> + /* Descriptor Ring Change Event Flags */
> + __virtio16 desc_event_flags;
Do you think it'd be better to remove the prefix (desc_event_) for
the fields. And it will be consistent with other definitions, e.g.:
struct vring_packed_desc {
/* Buffer Address. */
__virtio64 addr;
/* Buffer Length. */
__virtio32 len;
/* Buffer ID. */
__virtio16 id;
/* The flags depending on descriptor type. */
__virtio16 flags;
};
> +};
> +
> /* Virtio ring descriptors: 16 bytes. These can chain together via "next". */
> struct vring_desc {
> /* Address (guest-physical). */
> --
> 2.7.4
>
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