Re: [PATCH RFC 14/15] hw/virtio/vhost-user: handle data movement with shadow vqs

On 24.07.26 00:30, Connor Kite wrote:
> - Add start logic for shadow virtqueues, which sets vring addresses.
> - Update logic for sending vring addresses to backend to point
>    to the shadow vrings when isolation mode is active.
> - Implement handlers for intercepted avail and used descriptors. These
>    handlers copy buffer contents between bounce buffers in the isolation
>    region and the buffers made available by the guest
> - Implement logic to stop svqs
>
> Signed-off-by: Connor Kite <[email protected]>
> ---
>   hw/virtio/vhost-user.c | 167 ++++++++++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 166 insertions(+), 1 deletion(-)

In general, I find it strange to use two completely separate data 
structures for the address translation in either direction (avail/used). 
I suppose we could have a map of in-flight translations, adding entries 
in avail, and looking for them in used, removing them from the map.

I mean, sure, right now, we can calculate the addresses, because we 
“duplicate” the whole range of guest memory for each device, but that is 
clearly a waste and it would be better to use a smaller size in the 
future; and then each request will need to allocate from that smaller 
device memory, get a random address, and that will need such a map for 
tracking.

> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index 75858289a2..e65f877f9a 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -1299,6 +1299,97 @@ static int init_isolation_regions(struct vhost_dev *dev,
>       return 0;
>   }
>   
> +static int vhost_user_memory_lookup(struct vhost_dev *dev, hwaddr gpa,
> +                                    hwaddr *hva)
> +{
> +    int i;
> +    hwaddr offset;
> +
> +    for (i = 0; i < dev->mem->nregions; i++) {
> +        struct vhost_memory_region *reg = dev->mem->regions + i;
> +
> +        if (gpa >= reg->guest_phys_addr &&
> +            reg->guest_phys_addr + reg->memory_size > gpa) {
> +            offset = gpa - reg->guest_phys_addr;
> +            *hva = reg->userspace_addr + offset;
> +            return 0;
> +        }
> +    }
> +
> +    return -EFAULT;
> +}
> +
> +static int vhost_user_svq_handle_used(VhostShadowVirtqueue *svq,
> +                                      VirtQueueElement *elem,
> +                                      void *opaque)
> +{
> +    hwaddr hva;
> +    int r;
> +    struct vhost_dev *dev = opaque;
> +
> +    for (int i = 0; i < elem->in_num; i++) {
> +        r = vhost_user_memory_lookup(dev, elem->in_addr[i], &hva);

This function only looks for the starting address, so it is not 
guaranteed that the whole buffer is within one memory region.

> +        if (r < 0) {
> +            return r;
> +        }
> +
> +        memcpy((void *) hva, elem->in_sg[i].iov_base, elem->in_sg[i].iov_len);
> +        elem->in_sg[i].iov_base = (void *) hva;
> +    }
> +
> +    for (int i = 0; i < elem->out_num; i++) {
> +        r = vhost_user_memory_lookup(dev, elem->out_addr[i], &hva);
> +        if (r < 0) {
> +            return r;
> +        }
> +
> +        memcpy((void *) hva, elem->out_sg[i].iov_base, elem->out_sg[i].iov_len);

I don’t think we need this `memcpy()` here, as the device may only write 
to IN buffers, not OUT buffers.

> +        elem->out_sg[i].iov_base = (void *) hva;
> +    }
> +
> +    return 0;
> +}
> +
> +static int vhost_user_svq_handle_avail(VhostShadowVirtqueue *svq,
> +                                       VirtQueueElement *elem,
> +                                       void *opaque)
> +{
> +    hwaddr offset;
> +    const DMAMap *map;
> +    DMAMap needle;
> +    hwaddr *iova_base;
> +
> +    for (int i = 0; i < elem->out_num; i++) {
> +        needle.translated_addr = elem->out_addr[i];
> +        needle.size = elem->out_sg[i].iov_len - 1;
> +        map = vhost_iova_tree_find_gpa(svq->iova_tree, &needle);

Is it guaranteed that the returned `map` will fully contain `needle`?

> +        offset = needle.translated_addr - map->translated_addr;
> +        iova_base = (void *)(map->iova + offset);
> +
> +        elem->out_sg[i].iov_base = iova_base;
> +        memcpy(iova_base, elem->out_sg[i].iov_base, needle.size + 1);
> +    }
> +
> +    for (int i = 0; i < elem->in_num; i++) {
> +        needle.translated_addr = elem->in_addr[i];
> +        needle.size = elem->in_sg[i].iov_len - 1;
> +        map = vhost_iova_tree_find_gpa(svq->iova_tree, &needle);
> +        offset = needle.translated_addr - map->translated_addr;
> +        iova_base = (void *)(map->iova + offset);
> +
> +        elem->in_sg[i].iov_base = iova_base;
> +        memcpy(iova_base, elem->in_sg[i].iov_base, needle.size + 1);

I don’t think we need this `memcpy()` here as the device is to strictly 
write to IN buffers, not read from them.

> +    }
> +
> +    vhost_svq_add(svq, elem->out_sg, elem->out_num, elem->out_addr,
> +                  elem->in_sg, elem->in_num, elem->in_addr, elem);

Might make sense to make vhost_svq_add_element() public and use it here.

> +
> +    return 0;
> +}
> +
> +
> +
> +
>   static int vhost_user_set_mem_table(struct vhost_dev *dev,
>                                       struct vhost_memory *mem)
>   {
> @@ -1772,6 +1863,8 @@ static int vhost_user_set_vring_err(struct vhost_dev *dev,
>   static int vhost_user_set_vring_addr(struct vhost_dev *dev,
>                                        struct vhost_vring_addr *addr)
>   {
> +    struct vhost_user *u = dev->opaque;
> +
>       VhostUserMsg msg = {
>           .hdr.request = VHOST_USER_SET_VRING_ADDR,
>           .hdr.flags = VHOST_USER_VERSION,
> @@ -1779,6 +1872,25 @@ static int vhost_user_set_vring_addr(struct vhost_dev *dev,
>           .hdr.size = sizeof(msg.payload.addr),
>       };
>   
> +    if (u->user->memory_isolation) {
> +        if (!u->svqs_allocated) {
> +            return 0;
> +        }
> +
> +        int svq_idx = addr->index - dev->vq_index;
> +        VhostShadowVirtqueue *svq = g_ptr_array_index(u->shadow_vqs,
> +                                                      svq_idx);

Again, bounds checking via `vhost_user_get_vq_index()` would be good.

> +
> +        struct vhost_vring_addr svq_addr = {
> +            .avail_user_addr = (uint64_t)(uintptr_t)svq->vring.avail,
> +            .desc_user_addr = (uint64_t)(uintptr_t)svq->vring.desc,
> +            .used_user_addr = (uint64_t)(uintptr_t)svq->vring.used,
> +            .index = addr->index,
> +        };
> +
> +        msg.payload.addr = svq_addr;
> +    }
> +
>       /*
>        * wait for a reply if logging is enabled to make sure
>        * backend is actually logging changes
> @@ -2754,13 +2866,18 @@ static int vhost_user_postcopy_notifier(NotifierWithReturn *notifier,
>       return 0;
>   }
>   
> +static const VhostShadowVirtqueueOps vhost_user_svq_ops = {
> +    .avail_handler = vhost_user_svq_handle_avail,
> +    .used_handler = vhost_user_svq_handle_used
> +};
> +
>   static void vhost_user_init_svq(struct vhost_dev *dev, struct vhost_user *u)
>   {
>       /*Modified from vhost-vdpa*/
>       u->shadow_vqs = g_ptr_array_new_full(dev->nvqs, vhost_svq_free);
>       for (int i = 0; i < dev->nvqs; i++) {
>           VhostShadowVirtqueue *svq;
> -        svq = vhost_svq_new(NULL, NULL);
> +        svq = vhost_svq_new(&vhost_user_svq_ops, dev);
>           g_ptr_array_add(u->shadow_vqs, svq);
>       }
>   }
> @@ -3466,8 +3583,56 @@ void vhost_user_async_close(DeviceState *d,
>       }
>   }
>   
> +static bool vhost_user_svqs_start(struct vhost_dev *dev)
> +{
> +    struct vhost_user *u = dev->opaque;
> +    uint64_t vring_base = u->iso_memory.vring_base_addr;
> +    u->svqs_allocated = true;

I don’t like setting this before it actually being the case very much. I 
understand that it is necessary for how you decided to set up 
`vhost_user_set_vring_addr()`, but there should at least be a comment 
(because reading this code, there is the obvious question: Why is this 
set when it is not true?); or the code should be different. I.e. having 
a `vhost_user_set_shadow_vring_addr()` function, maybe, which would not 
even take an `addr` argument and could be used both below and in 
`vhost_user_set_vring_addr()`.

> +
> +    for (int i = 0; i < u->shadow_vqs->len; i++) {
> +        VirtQueue *vq = virtio_get_queue(dev->vdev, dev->vq_index + i);
> +        VhostShadowVirtqueue *svq = g_ptr_array_index(u->shadow_vqs, i);
> +        svq->base_addr = (hwaddr *) vring_base;

I really do not like the lack of bounds checking here again. There 
should be something, somewhere (i.e. in the “Isolated memory data” 
portion, maybe in `struct IsolationRegion`), that tells how much memory 
has been allocated for the shadow virtqueues, and we need to check that 
we do not overflow.

> +        vhost_svq_start(svq, dev->vdev, vq, u->iso_iova_tree);
> +
> +        struct vhost_vring_addr addr = {
> +            .index = dev->vq_index + i,
> +            .desc_user_addr = vring_base,
> +            .avail_user_addr = vring_base + sizeof(vring_desc_t) *
> +                svq->vring.num,
> +            .used_user_addr = vring_base + vhost_svq_driver_area_size(svq)

All values set here are effectively ignored, right? Because 
`vhost_user_set_vring_addr()` just overwrites them. (Having a 
`vhost_user_set_shadow_vring_addr()` could maybe save us from having to 
set this up.)

Not that it’s currently wrong to set them this way, just asking.

Hanna

> +        };
> +
> +        vhost_user_set_vring_addr(dev, &addr);
> +
> +        vring_base += vhost_svq_device_area_size(svq) +
> +                      vhost_svq_driver_area_size(svq);
> +    }
> +
> +    return false;
> +}
> +
> +static void vhost_user_svqs_stop(struct vhost_dev *dev)
> +{
> +    struct vhost_user *u = dev->opaque;
> +
> +    for (int i = 0; i < u->shadow_vqs->len; i++) {
> +        vhost_svq_stop(g_ptr_array_index(u->shadow_vqs, i));
> +    }
> +}
> +
> +
>   static int vhost_user_dev_start(struct vhost_dev *dev, bool started)
>   {
> +    struct vhost_user *u = dev->opaque;
> +    if (u->user->memory_isolation) {
> +        if (started) {
> +            vhost_user_svqs_start(dev);
> +        } else {
> +            vhost_user_svqs_stop(dev);
> +        }
> +    }
> +
>       if (!vhost_user_has_protocol_feature(dev, VHOST_USER_PROTOCOL_F_STATUS)) {
>           return 0;
>       }
>