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

Xu Zaibo
2018-05-25T02:39:33Z

Hi,

On 2018/5/24 23:04, Jean-Philippe Brucker wrote:
> 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.
>
I still don't understand why we should 'unbind_dev_all', is it possible
to do a 'unbind_dev_pasid'?
Then we can do other things instead of waiting that user may not like. :)

Thanks
Zaibo
>
>> +
>> __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. :)
>
>
>
> .
>