Re: [RFC v3 25/29] vhost: Add custom IOTLB translations to SVQ

[Jason Wang <jasowang@redhat.com>]

在 2021/5/20 上午12:28, Eugenio Pérez 写道:
> Use translations added in IOVAReverseMaps in SVQ if the vhost device
> does not support the mapping of the full qemu's virtual address space.
> In other cases, Shadow Virtqueue still uses the qemu's virtual address
> of the buffer pointed by the descriptor, which has been translated
> already by qemu's VirtQueue machinery.


I'd say let stick to a single kind of translation (iova allocator) that 
works for all the cases first and add optimizations on top.


>
> Now every element needs to store the previous address also, so VirtQueue
> can consume the elements properly. This adds a little overhead per VQ
> element, having to allocate more memory to stash them. As a possible
> optimization, this allocation could be avoided if the descriptor is not
> a chain but a single one, but this is left undone.
>
> Checking also for vhost_set_iotlb_callback to send used ring remapping.
> This is only needed for kernel, and would print an error in case of
> vhost devices with its own mapping (vdpa).
>
> This could change for other callback, like checking for
> vhost_force_iommu, enable_custom_iommu, or another. Another option could
> be to, at least, extract the check of "is map(used, writable) needed?"
> in another function. But at the moment just copy the check used in
> vhost_dev_start here.
>
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> ---
>   hw/virtio/vhost-shadow-virtqueue.c | 134 ++++++++++++++++++++++++++---
>   hw/virtio/vhost.c                  |  29 +++++--
>   2 files changed, 145 insertions(+), 18 deletions(-)
>
> diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
> index 934d3bb27b..a92da979d1 100644
> --- a/hw/virtio/vhost-shadow-virtqueue.c
> +++ b/hw/virtio/vhost-shadow-virtqueue.c
> @@ -10,12 +10,19 @@
>   #include "hw/virtio/vhost-shadow-virtqueue.h"
>   #include "hw/virtio/vhost.h"
>   #include "hw/virtio/virtio-access.h"
> +#include "hw/virtio/vhost-iova-tree.h"
>   
>   #include "standard-headers/linux/vhost_types.h"
>   
>   #include "qemu/error-report.h"
>   #include "qemu/main-loop.h"
>   
> +typedef struct SVQElement {
> +    VirtQueueElement elem;
> +    void **in_sg_stash;
> +    void **out_sg_stash;


Any reason for the trick like this?

Can we simply use iovec and iov_copy() here?

Thanks