Re: [PATCH v1] vhost-vdpa: Set discarding of RAM broken when initializing the backend

[Jason Wang]

On 2021/3/3 12:21 上午, David Hildenbrand wrote:
> Similar to VFIO, vDPA will go ahead an map+pin all guest memory. Memory
> that used to be discarded will get re-populated and if we
> discard+re-access memory after mapping+pinning, the pages mapped into the
> vDPA IOMMU will go out of sync with the actual pages mapped into the user
> space page tables.
>
> Set discarding of RAM broken such that:
> - virtio-mem and vhost-vdpa run mutually exclusive
> - virtio-balloon is inhibited and no memory discards will get issued
>
> In the future, we might be able to support coordinated discarding of RAM
> as used by virtio-mem and as planned for VFIO.
>
> Cc: Jason Wang <jasowang@redhat.com>
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Cc: Cindy Lu <lulu@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>


Acked-by: Jason Wang <jasowang@redhat.com>


> ---
>
> Note: I was not actually able to reproduce/test as I fail to get the
> vdpa_sim/vdpa_sim_net running on upstream Linux (whetever vdpa, vhost_vdpa,
> vdpa_sim, vdpa_sim_net modules I probe, and in which order, no vdpa devices
> appear under /sys/bus/vdpa/devices/ or /dev/).


The device creation was switched to use vdpa tool that is integrated 
with iproue2[1].

[1] 
https://git.kernel.org/pub/scm/network/iproute2/iproute2-next.git/commit/?id=143610383da51e1f868c6d5a2a5e2fb552293d18


> ---
>   hw/virtio/vhost-vdpa.c | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
>
> diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
> index 01d2101d09..86058d4041 100644
> --- a/hw/virtio/vhost-vdpa.c
> +++ b/hw/virtio/vhost-vdpa.c
> @@ -278,6 +278,17 @@ static int vhost_vdpa_init(struct vhost_dev *dev, void 
> *opaque)
>       uint64_t features;
>       assert(dev->vhost_ops->backend_type == VHOST_BACKEND_TYPE_VDPA);
>       trace_vhost_vdpa_init(dev, opaque);
> +    int ret;
> +
> +    /*
> +     * Similar to VFIO, we end up pinning all guest memory and have to
> +     * disable discarding of RAM.
> +     */
> +    ret = ram_block_discard_disable(true);
> +    if (ret) {
> +        error_report("Cannot set discarding of RAM broken");
> +        return ret;
> +    }


vDPA will support non pinning (shared VM) backend soon[2]. So I guess we 
need a flag to be advertised to usersapce then we can conditionly enable 
the discard here.

[2] https://www.spinics.net/lists/netdev/msg723944.html

Thanks


>   
>       v = opaque;
>       v->dev = dev;
> @@ -302,6 +313,8 @@ static int vhost_vdpa_cleanup(struct vhost_dev *dev)
>       memory_listener_unregister(&v->listener);
>   
>       dev->opaque = NULL;
> +    ram_block_discard_disable(false);
> +
>       return 0;
>   }
>