Re: [PATCH net V2 4/4] vhost: log dirty page correctly

Michael S. Tsirkin
2018-12-12T14:32:45Z

On Wed, Dec 12, 2018 at 06:08:19PM +0800, Jason Wang wrote:
> Vhost dirty page logging API is designed to sync through GPA. But we
> try to log GIOVA when device IOTLB is enabled. This is wrong and may
> lead to missing data after migration.
>
> To solve this issue, when logging with device IOTLB enabled, we will:
>
> 1) reuse the device IOTLB translation result of GIOVA->HVA mapping to
> get HVA, for writable descriptor, get HVA through iovec. For used
> ring update, translate its GIOVA to HVA
> 2) traverse the GPA->HVA mapping to get the possible GPA and log
> through GPA. Pay attention this reverse mapping is not guaranteed
> to be unique, so we should log each possible GPA in this case.
>
> This fix the failure of scp to guest during migration. In -next, we
> will probably support passing GIOVA->GPA instead of GIOVA->HVA.
>
> Fixes: 6b1e6cc7855b ("vhost: new device IOTLB API")
> Reported-by: Jintack Lim <jintack@cs.columbia.edu>
> Cc: Jintack Lim <jintack@cs.columbia.edu>
> Signed-off-by: Jason Wang <jasowang@redhat.com>

It's a nasty bug for sure but it's been like this for a long
time so I'm inclined to say let's put it in 4.21,
and queue for stable.

So please split this out from this series.

Also, I'd like to see a feature bit that allows GPA in IOTLBs.

> ---
> drivers/vhost/net.c | 3 +-
> drivers/vhost/vhost.c | 79 +++++++++++++++++++++++++++++++++++--------
> drivers/vhost/vhost.h | 3 +-
> 3 files changed, 69 insertions(+), 16 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index ad7a6f475a44..784df2b49628 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -1192,7 +1192,8 @@ static void handle_rx(struct vhost_net *net)
> if (nvq->done_idx > VHOST_NET_BATCH)
> vhost_net_signal_used(nvq);
> if (unlikely(vq_log))
> - vhost_log_write(vq, vq_log, log, vhost_len);
> + vhost_log_write(vq, vq_log, log, vhost_len,
> + vq->iov, in);
> total_len += vhost_len;
> if (unlikely(vhost_exceeds_weight(++recv_pkts, total_len))) {
> vhost_poll_queue(&vq->poll);
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 55e5aa662ad5..3660310604fd 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -1733,11 +1733,67 @@ static int log_write(void __user *log_base,
> return r;
> }
>
> +static int log_write_hva(struct vhost_virtqueue *vq, u64 hva, u64 len)
> +{
> + struct vhost_umem *umem = vq->umem;
> + struct vhost_umem_node *u;
> + u64 gpa;
> + int r;
> + bool hit = false;
> +
> + list_for_each_entry(u, &umem->umem_list, link) {
> + if (u->userspace_addr < hva &&
> + u->userspace_addr + u->size >=
> + hva + len) {
> + gpa = u->start + hva - u->userspace_addr;
> + r = log_write(vq->log_base, gpa, len);
> + if (r < 0)
> + return r;
> + hit = true;
> + }
> + }
> +
> + /* No reverse mapping, should be a bug */
> + WARN_ON(!hit);

Maybe it should but userspace can trigger this easily I think.
We need to stop the device not warn in kernel log.

Also there's an error fd: VHOST_SET_VRING_ERR, need to wake it up.

> + return 0;
> +}
> +
> +static void log_used(struct vhost_virtqueue *vq, u64 used_offset, u64 len)
> +{
> + struct iovec iov[64];
> + int i, ret;
> +
> + if (!vq->iotlb) {
> + log_write(vq->log_base, vq->log_addr + used_offset, len);
> + return;
> + }

This change seems questionable. used ring writes
use their own machinery it does not go through iotlb.
Same should apply to log I think.

> +
> + ret = translate_desc(vq, (u64)(uintptr_t)vq->used + used_offset,
> + len, iov, 64, VHOST_ACCESS_WO);
> + WARN_ON(ret < 0);

Same thing here. translation failures can be triggered from guest.
warn on is not a good error handling strategy ...

> +
> + for (i = 0; i < ret; i++) {
> + ret = log_write_hva(vq, (u64)(uintptr_t)iov[i].iov_base,
> + iov[i].iov_len);
> + WARN_ON(ret);
> + }
> +}
> +
> int vhost_log_write(struct vhost_virtqueue *vq, struct vhost_log *log,
> - unsigned int log_num, u64 len)
> + unsigned int log_num, u64 len, struct iovec *iov, int count)
> {
> int i, r;
>
> + if (vq->iotlb) {
> + for (i = 0; i < count; i++) {
> + r = log_write_hva(vq, (u64)(uintptr_t)iov[i].iov_base,
> + iov[i].iov_len);
> + if (r < 0)
> + return r;
> + }
> + return 0;
> + }
> +
> /* Make sure data written is seen before log. */
> smp_wmb();
> for (i = 0; i < log_num; ++i) {
> @@ -1769,9 +1825,8 @@ static int vhost_update_used_flags(struct vhost_virtqueue *vq)
> smp_wmb();
> /* Log used flag write. */
> used = &vq->used->flags;
> - log_write(vq->log_base, vq->log_addr +
> - (used - (void __user *)vq->used),
> - sizeof vq->used->flags);
> + log_used(vq, (used - (void __user *)vq->used),
> + sizeof vq->used->flags);
> if (vq->log_ctx)
> eventfd_signal(vq->log_ctx, 1);
> }
> @@ -1789,9 +1844,8 @@ static int vhost_update_avail_event(struct vhost_virtqueue *vq, u16 avail_event)
> smp_wmb();
> /* Log avail event write */
> used = vhost_avail_event(vq);
> - log_write(vq->log_base, vq->log_addr +
> - (used - (void __user *)vq->used),
> - sizeof *vhost_avail_event(vq));
> + log_used(vq, (used - (void __user *)vq->used),
> + sizeof *vhost_avail_event(vq));
> if (vq->log_ctx)
> eventfd_signal(vq->log_ctx, 1);
> }
> @@ -2191,10 +2245,8 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq,
> /* Make sure data is seen before log. */
> smp_wmb();
> /* Log used ring entry write. */
> - log_write(vq->log_base,
> - vq->log_addr +
> - ((void __user *)used - (void __user *)vq->used),
> - count * sizeof *used);
> + log_used(vq, ((void __user *)used - (void __user *)vq->used),
> + count * sizeof *used);
> }
> old = vq->last_used_idx;
> new = (vq->last_used_idx += count);
> @@ -2236,9 +2288,8 @@ int vhost_add_used_n(struct vhost_virtqueue *vq, struct vring_used_elem *heads,
> /* Make sure used idx is seen before log. */
> smp_wmb();
> /* Log used index update. */
> - log_write(vq->log_base,
> - vq->log_addr + offsetof(struct vring_used, idx),
> - sizeof vq->used->idx);
> + log_used(vq, offsetof(struct vring_used, idx),
> + sizeof vq->used->idx);
> if (vq->log_ctx)
> eventfd_signal(vq->log_ctx, 1);
> }
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 466ef7542291..1b675dad5e05 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -205,7 +205,8 @@ bool vhost_vq_avail_empty(struct vhost_dev *, struct vhost_virtqueue *);
> bool vhost_enable_notify(struct vhost_dev *, struct vhost_virtqueue *);
>
> int vhost_log_write(struct vhost_virtqueue *vq, struct vhost_log *log,
> - unsigned int log_num, u64 len);
> + unsigned int log_num, u64 len,
> + struct iovec *iov, int count);
> int vq_iotlb_prefetch(struct vhost_virtqueue *vq);
>
> struct vhost_msg_node *vhost_new_msg(struct vhost_virtqueue *vq, int type);
> --
> 2.17.1