Re: [Qemu-devel] [PATCH qemu v16 01/19] vfio: Delay DMA address space listener release

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 05/06/2016 08:39 AM, Alex Williamson wrote:
> On Wed,  4 May 2016 16:52:13 +1000
> Alexey Kardashevskiy <aik@ozlabs.ru> wrote:
>
>> This postpones VFIO container deinitialization to let region_del()
>> callbacks (called via vfio_listener_release) do proper clean up
>> while the group is still attached to the container.
>
> Any mappings within the container should clean themselves up when the
> container is deprivleged by removing the last group in the kernel. Is
> the issue that that doesn't happen, which would be a spapr vfio kernel
> bug, or that our QEMU side structures get all out of whack if we let
> that happen?

My mailbase got corrupted, missed that.

This is mostly for "[PATCH qemu v16 17/19] spapr_iommu, vfio, memory: 
Notify IOMMU about starting/stopping being used by VFIO", I should have put 
01/19 and 02/19 right before 17/19, sorry about that.


Every reboot the spapr machine removes all (i.e. one or two) windows and 
creates the default one.

I do this by memory_region_del_subregion(iommu_mr) + 
memory_region_add_subregion(iommu_mr). Which gets translated to 
VFIO_IOMMU_SPAPR_TCE_REMOVE + VFIO_IOMMU_SPAPR_TCE_CREATE via 
vfio_memory_listener if there is VFIO; no direct calls from spapr to vfio 
=> cool. During the machine reset, the VFIO device is there with the 
container and groups attached, at some point with no windows.

Now to VFIO plug/unplug.

When VFIO plug happens, vfio_memory_listener is created, region_add() is 
called, the hardware window is created (via VFIO_IOMMU_SPAPR_TCE_CREATE).
Unplugging should end up doing VFIO_IOMMU_SPAPR_TCE_REMOVE somehow. If 
region_del() is not called when the container is being destroyed (as before 
this patchset), then the kernel cleans and destroys windows when 
close(container->fd) is called or when qemu is killed (and this fd is 
naturally closed), I hope this answers the comment from 02/19.

So far so good (right?)

However I also have a guest view of the TCE table, this is what the guest 
sees and this is what emulated PCI devices use. This guest view is either 
allocated in the KVM (so H_PUT_TCE can be handled quickly right in the host 
kernel, even in real mode) or userspace (VFIO case).

I generally want the guest view to be in the KVM. However when I plug VFIO, 
I have to move the table to the userspace. When I unplug VFIO, I want to do 
the opposite so I need a way to tell spapr that it can move the table. 
region_del() seemed a natural way of doing this as region_add() is already 
doing the opposite part.

With this patchset, each IOMMU MR gets a usage counter, region_add() does 
+1, region_del() does -1 (yeah, not extremely optimal during reset). When 
the counter goes from 0 to 1, vfio_start() hook is called, when the counter 
becomes 0 - vfio_stop(). Note that we may have multiple VFIO containers on 
the same PHB.

Without 01/19 and 02/19, I'll have to repeat region_del()'s counter 
decrement steps in vfio_disconnect_container(). And I still cannot move 
counting from region_add() to vfio_connect_container() so there will be 
asymmetry which I am fine with, I am just checking here - what would be the 
best approach here?


Thanks.



>
>>
>> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
>> ---
>>  hw/vfio/common.c | 22 +++++++++++++++-------
>>  1 file changed, 15 insertions(+), 7 deletions(-)
>>
>> diff --git a/hw/vfio/common.c b/hw/vfio/common.c
>> index fe5ec6a..0b40262 100644
>> --- a/hw/vfio/common.c
>> +++ b/hw/vfio/common.c
>> @@ -921,23 +921,31 @@ static void vfio_disconnect_container(VFIOGroup *group)
>>  {
>>      VFIOContainer *container = group->container;
>>
>> -    if (ioctl(group->fd, VFIO_GROUP_UNSET_CONTAINER, &container->fd)) {
>> -        error_report("vfio: error disconnecting group %d from container",
>> -                     group->groupid);
>> -    }
>> -
>>      QLIST_REMOVE(group, container_next);
>> +
>> +    if (QLIST_EMPTY(&container->group_list)) {
>> +        VFIOGuestIOMMU *giommu;
>> +
>> +        vfio_listener_release(container);
>> +
>> +        QLIST_FOREACH(giommu, &container->giommu_list, giommu_next) {
>> +            memory_region_unregister_iommu_notifier(&giommu->n);
>> +        }
>> +    }
>> +
>>      group->container = NULL;
>> +    if (ioctl(group->fd, VFIO_GROUP_UNSET_CONTAINER, &container->fd)) {
>> +        error_report("vfio: error disconnecting group %d from container",
>> +                     group->groupid);
>> +    }
>>
>>      if (QLIST_EMPTY(&container->group_list)) {
>>          VFIOAddressSpace *space = container->space;
>>          VFIOGuestIOMMU *giommu, *tmp;
>>
>> -        vfio_listener_release(container);
>>          QLIST_REMOVE(container, next);
>>
>>          QLIST_FOREACH_SAFE(giommu, &container->giommu_list, giommu_next, tmp) {
>> -            memory_region_unregister_iommu_notifier(&giommu->n);
>>              QLIST_REMOVE(giommu, giommu_next);
>>              g_free(giommu);
>>          }
>
> I'm not spotting why this is a 2-pass process vs simply moving the
> existing QLIST_EMPTY cleanup above the ioctl.  Thanks,





-- 
Alexey