Re: [PATCH v7 1/3] vhost: invalidate vring access on IOTLB transitions

[Stefano Garzarella <sgarzare@redhat.com>]

On Thu, Aug 20, 2026 at 04:03:30PM +0800, Jia Jia wrote:
>When ACCESS_PLATFORM changes, the addresses cached in desc, avail, and
>used change meaning with the address space. Clear the cached vring access
>state when the device IOTLB is installed or removed so stale IOVAs cannot
>be reused as direct userspace addresses.
>
>Keep device IOTLB initialization idempotent and apply the mode change even
>when a virtqueue backend is attached. Drop the device-wide IOTLB first,
>then clear each VQ state under its own mutex, and keep the old table alive
>until every VQ has completed the handoff.
>
>A successful live mode change leaves the backend attached but invalidates
>the cached vring addresses. Userspace must configure the vring addresses
>for the new address mode before data processing can resume.
>
>Fixes: 6b1e6cc7855b ("vhost: new device IOTLB API")
>Signed-off-by: Jia Jia <physicalmtea@gmail.com>
>---
> drivers/vhost/vhost.c | 53 ++++++++++++++++++++++++++++++++++++++++++-
> drivers/vhost/vhost.h |  1 +
> 2 files changed, 53 insertions(+), 1 deletion(-)
>
>diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>index 14637cff0bd4..31fff9800045 100644
>--- a/drivers/vhost/vhost.c
>+++ b/drivers/vhost/vhost.c
>@@ -344,6 +344,17 @@ static void __vhost_vq_meta_reset(struct vhost_virtqueue *vq)
> 		vq->meta_iotlb[j] = NULL;
> }
>
>+/* Caller must hold the virtqueue mutex. */
>+static void vhost_vq_invalidate_access(struct vhost_virtqueue *vq)
>+{
>+	vq->desc = NULL;
>+	vq->avail = NULL;
>+	vq->used = NULL;
>+	vq->log_used = false;
>+	vq->log_addr = -1ull;
>+	__vhost_vq_meta_reset(vq);
>+}
>+
> static void vhost_vq_meta_reset(struct vhost_dev *d)
> {
> 	int i;
>@@ -1918,6 +1929,9 @@ int vq_meta_prefetch(struct vhost_virtqueue *vq)
> {
> 	unsigned int num = vq->num;
>
>+	if (!vq->desc || !vq->avail || !vq->used)
>+		return 0;
>+
> 	if (!vq->iotlb)
> 		return 1;
>
>@@ -2287,11 +2301,48 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
> }
> EXPORT_SYMBOL_GPL(vhost_vring_ioctl);
>
>+/* Caller must hold the device mutex. */
>+void vhost_clear_device_iotlb(struct vhost_dev *d)

Is this the right patch where introduce this function?

IMO should be introduced when we use it.

About that I'm not sure if it is better to squash the other 2 patches 
with this one, otherwise will be this bisectable?
I mean with just this patch applied (and without the other 2) who is 
going to free the old iotlb?

>+{
>+	struct vhost_iotlb *iotlb;
>+	int i;
>+
>+	iotlb = d->iotlb;
>+	if (!iotlb)
>+		return;
>+
>+	/*
>+	 * Drop the device-wide view first.  Each VQ then drops its
>+	 * per-VQ view and its cached ring access under its own mutex.
>+	 * Keep the old table alive until every VQ has completed this
>+	 * handoff, since a worker may still be using it while waiting
>+	 * for its VQ mutex.
>+	 */
>+	d->iotlb = NULL;
>+
>+	for (i = 0; i < d->nvqs; ++i) {
>+		struct vhost_virtqueue *vq = d->vqs[i];
>+
>+		mutex_lock(&vq->mutex);
>+		vq->iotlb = NULL;
>+		vhost_vq_invalidate_access(vq);
>+		mutex_unlock(&vq->mutex);
>+	}
>+
>+	vhost_clear_msg(d);
>+	vhost_iotlb_free(iotlb);
>+	wake_up_interruptible_poll(&d->wait, EPOLLIN | EPOLLRDNORM);
>+}
>+EXPORT_SYMBOL_GPL(vhost_clear_device_iotlb);
>+
> int vhost_init_device_iotlb(struct vhost_dev *d)
> {
> 	struct vhost_iotlb *niotlb, *oiotlb;
> 	int i;
>
>+	if (d->iotlb)
>+		return 0;
>+

IIUC after this patch `oiotlb` is always NULL, can we remove it?

Thanks,
Stefano

> 	if (max_iotlb_entries <= 0)
> 		return -EINVAL;
>
>@@ -2307,7 +2358,7 @@ int vhost_init_device_iotlb(struct vhost_dev *d)
>
> 		mutex_lock(&vq->mutex);
> 		vq->iotlb = niotlb;
>-		__vhost_vq_meta_reset(vq);
>+		vhost_vq_invalidate_access(vq);
> 		mutex_unlock(&vq->mutex);
> 	}
>
>diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
>index 0192ade6e749..3c75e8089373 100644
>--- a/drivers/vhost/vhost.h
>+++ b/drivers/vhost/vhost.h
>@@ -277,6 +277,7 @@ ssize_t vhost_chr_read_iter(struct vhost_dev *dev, struct iov_iter *to,
> 			    int noblock);
> ssize_t vhost_chr_write_iter(struct vhost_dev *dev,
> 			     struct iov_iter *from);
>+void vhost_clear_device_iotlb(struct vhost_dev *d);
> int vhost_init_device_iotlb(struct vhost_dev *d);
>
> void vhost_iotlb_map_free(struct vhost_iotlb *iotlb,
>-- 
>2.34.1
>