On Sun, Aug 11, 2024 at 7:20 PM Sahil <icegambi...@gmail.com> wrote:
>
> Hi,
>
> On Wednesday, August 7, 2024 9:52:10 PM GMT+5:30 Eugenio Perez Martin wrote:
> > On Fri, Aug 2, 2024 at 1:22 PM Sahil Siddiq <icegambi...@gmail.com> wrote:
> > > [...]
> > > @@ -726,17 +738,30 @@ void vhost_svq_start(VhostShadowVirtqueue *svq,
> > > VirtIODevice *vdev,
> > > svq->vring.num = virtio_queue_get_num(vdev,
> > > virtio_get_queue_index(vq));
> > > svq->num_free = svq->vring.num;
> > >
> > > - svq->vring.desc = mmap(NULL, vhost_svq_driver_area_size(svq),
> > > - PROT_READ | PROT_WRITE, MAP_SHARED |
> > > MAP_ANONYMOUS,
> > > - -1, 0);
> > > - desc_size = sizeof(vring_desc_t) * svq->vring.num;
> > > - svq->vring.avail = (void *)((char *)svq->vring.desc + desc_size);
> > > - svq->vring.used = mmap(NULL, vhost_svq_device_area_size(svq),
> > > - PROT_READ | PROT_WRITE, MAP_SHARED |
> > > MAP_ANONYMOUS,
> > > - -1, 0);
> > > - svq->desc_state = g_new0(SVQDescState, svq->vring.num);
> > > - svq->desc_next = g_new0(uint16_t, svq->vring.num);
> > > - for (unsigned i = 0; i < svq->vring.num - 1; i++) {
> > > + svq->is_packed = virtio_vdev_has_feature(svq->vdev,
> > > VIRTIO_F_RING_PACKED);
> > > +
> > > + if (virtio_vdev_has_feature(svq->vdev, VIRTIO_F_RING_PACKED)) {
> > > + svq->vring_packed.vring.desc = mmap(NULL,
> > > vhost_svq_memory_packed(svq),
> > > + PROT_READ | PROT_WRITE,
> > > MAP_SHARED | MAP_ANONYMOUS,
> > > + -1, 0);
> > > + desc_size = sizeof(struct vring_packed_desc) * svq->vring.num;
> > > + svq->vring_packed.vring.driver = (void *)((char
> > > *)svq->vring_packed.vring.desc + desc_size);
> > > + svq->vring_packed.vring.device = (void *)((char
> > > *)svq->vring_packed.vring.driver +
> > > + sizeof(struct
> > > vring_packed_desc_event));
> >
> > This is a great start but it will be problematic when you start
> > mapping the areas to the vdpa device. The driver area should be read
> > only for the device, but it is placed in the same page as a RW one.
> >
> > More on this later.
> >
> > > + } else {
> > > + svq->vring.desc = mmap(NULL, vhost_svq_driver_area_size(svq),
> > > + PROT_READ | PROT_WRITE, MAP_SHARED
> > > |MAP_ANONYMOUS,
> > > + -1, 0);
> > > + desc_size = sizeof(vring_desc_t) * svq->vring.num;
> > > + svq->vring.avail = (void *)((char *)svq->vring.desc + desc_size);
> > > + svq->vring.used = mmap(NULL, vhost_svq_device_area_size(svq),
> > > + PROT_READ | PROT_WRITE, MAP_SHARED
> > > |MAP_ANONYMOUS,
> > > + -1, 0);
> > > + }
> >
> > I think it will be beneficial to avoid "if (packed)" conditionals on
> > the exposed functions that give information about the memory maps.
> > These need to be replicated at
> > hw/virtio/vhost-vdpa.c:vhost_vdpa_svq_map_rings.
> >
> > However, the current one depends on the driver area to live in the
> > same page as the descriptor area, so it is not suitable for this.
>
> I haven't really understood this.
>
> In split vqs the descriptor, driver and device areas are mapped to RW pages.
> In vhost_vdpa.c:vhost_vdpa_svq_map_rings, the regions are mapped with
> the appropriate "perm" field that sets the R/W permissions in the DMAMap
> object. Is this problematic for the split vq format because the avail ring is
> anyway mapped to a RW page in "vhost_svq_start"?
>
Ok so maybe the map word is misleading here. The pages needs to be
allocated for the QEMU process with both PROT_READ | PROT_WRITE, as
QEMU needs to write into it.
They are mapped to the device with vhost_vdpa_dma_map, and the last
bool parameter indicates if the device needs write permissions or not.
You can see how hw/virtio/vhost-vdpa.c:vhost_vdpa_svq_map_ring checks
the needle permission for this, and the needle permissions are stored
at hw/virtio/vhost-vdpa.c:vhost_vdpa_svq_map_rings. This is the
function that needs to check for the maps permissions.
> For packed vqs, the "Driver Event Suppression" data structure should be
> read-only for the device. Similar to split vqs, this is mapped to a RW page
> in "vhost_svq_start" but it is then mapped to a DMAMap object with read-
> only perms in "vhost_vdpa_svq_map_rings".
>
> I am a little confused about where the issue lies.
>
> Thanks,
> Sahil
>
>
