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Date: Mon, 23 Aug 2021 17:19:56 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Vincent Whitchurch <vincent.whitchurch@...s.com>
Cc: Jason Wang <jasowang@...hat.com>, kernel@...s.com,
kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vhost: add support for mandatory barriers
On Mon, Aug 23, 2021 at 10:14:37AM +0200, Vincent Whitchurch wrote:
> vhost always uses SMP-conditional barriers, but these may not be
> sufficient when vhost is used to communicate between heterogeneous
> processors in an AMP configuration, especially since they're NOPs on
> !SMP builds.
>
> To solve this, use the virtio_*() barrier functions and ask them for
> non-weak barriers if requested by userspace.
>
> Signed-off-by: Vincent Whitchurch <vincent.whitchurch@...s.com>
I am inclined to say let's (ab)use VIRTIO_F_ORDER_PLATFORM for this.
Jason what do you think?
Also is the use of DMA variants really the intended thing here? Could
you point me at some examples please?
> ---
> drivers/vhost/vhost.c | 23 ++++++++++++++---------
> drivers/vhost/vhost.h | 2 ++
> include/uapi/linux/vhost.h | 2 ++
> 3 files changed, 18 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index b9e853e6094d..f7172e1bc395 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -500,6 +500,7 @@ void vhost_dev_init(struct vhost_dev *dev,
> vq->indirect = NULL;
> vq->heads = NULL;
> vq->dev = dev;
> + vq->weak_barriers = true;
> mutex_init(&vq->mutex);
> vhost_vq_reset(dev, vq);
> if (vq->handle_kick)
> @@ -1801,6 +1802,10 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
> if (ctx)
> eventfd_ctx_put(ctx);
> break;
> + case VHOST_SET_STRONG_BARRIERS:
> + for (i = 0; i < d->nvqs; ++i)
> + d->vqs[i]->weak_barriers = false;
> + break;
> default:
> r = -ENOIOCTLCMD;
> break;
> @@ -1927,7 +1932,7 @@ int vhost_log_write(struct vhost_virtqueue *vq, struct vhost_log *log,
> int i, r;
>
> /* Make sure data written is seen before log. */
> - smp_wmb();
> + virtio_wmb(vq->weak_barriers);
>
> if (vq->iotlb) {
> for (i = 0; i < count; i++) {
> @@ -1964,7 +1969,7 @@ static int vhost_update_used_flags(struct vhost_virtqueue *vq)
> return -EFAULT;
> if (unlikely(vq->log_used)) {
> /* Make sure the flag is seen before log. */
> - smp_wmb();
> + virtio_wmb(vq->weak_barriers);
> /* Log used flag write. */
> used = &vq->used->flags;
> log_used(vq, (used - (void __user *)vq->used),
> @@ -1982,7 +1987,7 @@ static int vhost_update_avail_event(struct vhost_virtqueue *vq, u16 avail_event)
> if (unlikely(vq->log_used)) {
> void __user *used;
> /* Make sure the event is seen before log. */
> - smp_wmb();
> + virtio_wmb(vq->weak_barriers);
> /* Log avail event write */
> used = vhost_avail_event(vq);
> log_used(vq, (used - (void __user *)vq->used),
> @@ -2228,7 +2233,7 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> /* Only get avail ring entries after they have been
> * exposed by guest.
> */
> - smp_rmb();
> + virtio_rmb(vq->weak_barriers);
> }
>
> /* Grab the next descriptor number they're advertising, and increment
> @@ -2367,7 +2372,7 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq,
> }
> if (unlikely(vq->log_used)) {
> /* Make sure data is seen before log. */
> - smp_wmb();
> + virtio_wmb(vq->weak_barriers);
> /* Log used ring entry write. */
> log_used(vq, ((void __user *)used - (void __user *)vq->used),
> count * sizeof *used);
> @@ -2402,14 +2407,14 @@ int vhost_add_used_n(struct vhost_virtqueue *vq, struct vring_used_elem *heads,
> r = __vhost_add_used_n(vq, heads, count);
>
> /* Make sure buffer is written before we update index. */
> - smp_wmb();
> + virtio_wmb(vq->weak_barriers);
> if (vhost_put_used_idx(vq)) {
> vq_err(vq, "Failed to increment used idx");
> return -EFAULT;
> }
> if (unlikely(vq->log_used)) {
> /* Make sure used idx is seen before log. */
> - smp_wmb();
> + virtio_wmb(vq->weak_barriers);
> /* Log used index update. */
> log_used(vq, offsetof(struct vring_used, idx),
> sizeof vq->used->idx);
> @@ -2428,7 +2433,7 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
> /* Flush out used index updates. This is paired
> * with the barrier that the Guest executes when enabling
> * interrupts. */
> - smp_mb();
> + virtio_mb(vq->weak_barriers);
>
> if (vhost_has_feature(vq, VIRTIO_F_NOTIFY_ON_EMPTY) &&
> unlikely(vq->avail_idx == vq->last_avail_idx))
> @@ -2530,7 +2535,7 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
> }
> /* They could have slipped one in as we were doing that: make
> * sure it's written, then check again. */
> - smp_mb();
> + virtio_mb(vq->weak_barriers);
> r = vhost_get_avail_idx(vq, &avail_idx);
> if (r) {
> vq_err(vq, "Failed to check avail idx at %p: %d\n",
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 638bb640d6b4..5bd20d0db457 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -108,6 +108,8 @@ struct vhost_virtqueue {
> bool log_used;
> u64 log_addr;
>
> + bool weak_barriers;
> +
> struct iovec iov[UIO_MAXIOV];
> struct iovec iotlb_iov[64];
> struct iovec *indirect;
> diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> index c998860d7bbc..4b8656307f51 100644
> --- a/include/uapi/linux/vhost.h
> +++ b/include/uapi/linux/vhost.h
> @@ -97,6 +97,8 @@
> #define VHOST_SET_BACKEND_FEATURES _IOW(VHOST_VIRTIO, 0x25, __u64)
> #define VHOST_GET_BACKEND_FEATURES _IOR(VHOST_VIRTIO, 0x26, __u64)
>
> +#define VHOST_SET_STRONG_BARRIERS _IO(VHOST_VIRTIO, 0x27)
> +
> /* VHOST_NET specific defines */
>
> /* Attach virtio net ring to a raw socket, or tap device.
> --
> 2.28.0
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