Re: [PATCH QEMU v25 13/17] vfio: create mapped iova list when vIOMMU is enabled

[Kirti Wankhede]

On 6/25/2020 12:25 AM, Alex Williamson wrote:
> On Sun, 21 Jun 2020 01:51:22 +0530
> Kirti Wankhede <kwankhede@nvidia.com> wrote:
>
> > Create mapped iova list when vIOMMU is enabled. For each mapped iova
> > save translated address. Add node to list on MAP and remove node from
> > list on UNMAP.
> > This list is used to track dirty pages during migration.
>
> This seems like a lot of overhead to support that the VM might migrate.
> Is there no way we can build this when we start migration, for example
> replaying the mappings at that time?  Thanks,

In my previous version I tried to go through whole range and find valid 
iotlb, as below:

+        if (memory_region_is_iommu(section->mr)) {
+            iotlb = address_space_get_iotlb_entry(container->space->as, 
iova,
+                                                 true, 
MEMTXATTRS_UNSPECIFIED);

When mapping doesn't exist, qemu throws error as below:

qemu-system-x86_64: vtd_iova_to_slpte: detected slpte permission error 
(iova=0x0, level=0x3, slpte=0x0, write=1)
qemu-system-x86_64: vtd_iommu_translate: detected translation failure 
(dev=00:03:00, iova=0x0)
qemu-system-x86_64: New fault is not recorded due to compression of faults

Secondly, it iterates through whole range with IOMMU page size 
granularity which is 4K, so it takes long time resulting in large 
downtime. With this optimization, downtime with vIOMMU reduced 
significantly.

Other option I will try if I can check that if migration is supported 
then only create this list.

Thanks,
Kirti