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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 12 Dec 2018 09:32:45 -0500

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 <[email-masked]>
> Cc: Jintack Lim <[email-masked]>
> Signed-off-by: Jason Wang <[email-masked]>

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