Re: [PATCH v2 13/40] vfio: Add support for Shared Virtual Addressing

Jean-Philippe Brucker
2018-05-24T15:04:23Z

On 24/05/18 13:35, Xu Zaibo wrote:
>> Right, sva_init() must be called once for any device that intends to use
>> bind(). For the second process though, group->sva_enabled will be true
>> so we won't call sva_init() again, only bind().
>
> Well, while I create mediated devices based on one parent device to support multiple
> processes(a new process will create a new 'vfio_group' for the corresponding mediated device,
> and 'sva_enabled' cannot work any more), in fact, *sva_init and *sva_shutdown are basically
> working on parent device, so, as a result, I just only need sva initiation and shutdown on the
> parent device only once. So I change the two as following:
>
> @@ -551,8 +565,18 @@ int iommu_sva_device_init(struct device *dev, unsigned long features,
> if (features & ~IOMMU_SVA_FEAT_IOPF)
> return -EINVAL;
>
> + /* If already exists, do nothing */
> + mutex_lock(&dev->iommu_param->lock);
> + if (dev->iommu_param->sva_param) {
> + mutex_unlock(&dev->iommu_param->lock);
> + return 0;
> + }
> + mutex_unlock(&dev->iommu_param->lock);
>
> if (features & IOMMU_SVA_FEAT_IOPF) {
> ret = iommu_register_device_fault_handler(dev, iommu_queue_iopf,
>
>
> @@ -621,6 +646,14 @@ int iommu_sva_device_shutdown(struct device *dev)
> if (!domain)
> return -ENODEV;
>
> + /* If any other process is working on the device, shut down does nothing. */
> + mutex_lock(&dev->iommu_param->lock);
> + if (!list_empty(&dev->iommu_param->sva_param->mm_list)) {
> + mutex_unlock(&dev->iommu_param->lock);
> + return 0;
> + }
> + mutex_unlock(&dev->iommu_param->lock);

I don't think iommu-sva.c is the best place for this, it's probably
better to implement an intermediate layer (the mediating driver), that
calls iommu_sva_device_init() and iommu_sva_device_shutdown() once. Then
vfio-pci would still call these functions itself, but for mdev the
mediating driver keeps a refcount of groups, and calls device_shutdown()
only when freeing the last mdev.

A device driver (non mdev in this example) expects to be able to free
all its resources after sva_device_shutdown() returns. Imagine the
mm_list isn't empty (mm_exit() is running late), and instead of waiting
in unbind_dev_all() below, we return 0 immediately. Then the calling
driver frees its resources, and the mm_exit callback along with private
data passed to bind() disappear. If a mm_exit() is still running in
parallel, then it will try to access freed data and corrupt memory. So
in this function if mm_list isn't empty, the only thing we can do is wait.

Thanks,
Jean

> +
> __iommu_sva_unbind_dev_all(dev);
>
> mutex_lock(&dev->iommu_param->lock);
>
> I add the above two checkings in both *sva_init and *sva_shutdown, it is working now,
> but i don't know if it will cause any new problems. What's more, i doubt if it is
> reasonable to check this to avoid repeating operation in VFIO, maybe it is better to check
> in IOMMU. :)