On Tue, Oct 17, 2023 at 4:30 AM Si-Wei Liu [off-list ref] wrote:

On 10/16/2023 4:28 AM, Eugenio Perez Martin wrote:

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On Mon, Oct 16, 2023 at 8:33 AM Jason Wang [off-list ref] wrote:

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On Fri, Oct 13, 2023 at 3:36 PM Si-Wei Liu [off-list ref] wrote:

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On 10/12/2023 8:01 PM, Jason Wang wrote:

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On Tue, Oct 10, 2023 at 5:05 PM Si-Wei Liu [off-list ref] wrote:

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Devices with on-chip IOMMU or vendor specific IOTLB implementation
may need to restore iotlb mapping to the initial or default state
using the .reset_map op, as it's desirable for some parent devices
to solely manipulate mappings by its own, independent of virtio device
state. For instance, device reset does not cause mapping go away on
such IOTLB model in need of persistent mapping. Before vhost-vdpa
is going away, give them a chance to reset iotlb back to the initial
state in vhost_vdpa_cleanup().

Signed-off-by: Si-Wei Liu <redacted>
---
   drivers/vhost/vdpa.c | 16 ++++++++++++++++
   1 file changed, 16 insertions(+)

diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
index 851535f..a3f8160 100644
--- a/drivers/vhost/vdpa.c
+++ b/drivers/vhost/vdpa.c

@@ -131,6 +131,15 @@ static struct vhost_vdpa_as *vhost_vdpa_find_alloc_as(struct vhost_vdpa *v,
          return vhost_vdpa_alloc_as(v, asid);
   }

+static void vhost_vdpa_reset_map(struct vhost_vdpa *v, u32 asid)
+{
+       struct vdpa_device *vdpa = v->vdpa;
+       const struct vdpa_config_ops *ops = vdpa->config;
+
+       if (ops->reset_map)
+               ops->reset_map(vdpa, asid);
+}
+
   static int vhost_vdpa_remove_as(struct vhost_vdpa *v, u32 asid)
   {
          struct vhost_vdpa_as *as = asid_to_as(v, asid);

@@ -140,6 +149,13 @@ static int vhost_vdpa_remove_as(struct vhost_vdpa *v, u32 asid)

          hlist_del(&as->hash_link);
          vhost_vdpa_iotlb_unmap(v, &as->iotlb, 0ULL, 0ULL - 1, asid);
+       /*
+        * Devices with vendor specific IOMMU may need to restore
+        * iotlb to the initial or default state which is not done
+        * through device reset, as the IOTLB mapping manipulation
+        * could be decoupled from the virtio device life cycle.
+        */

Should we do this according to whether IOTLB_PRESIST is set?

Well, in theory this seems like so but it's unnecessary code change
actually, as that is the way how vDPA parent behind platform IOMMU works
today, and userspace doesn't break as of today. :)

Well, this is one question I've ever asked before. You have explained
that one of the reason that we don't break userspace is that they may
couple IOTLB reset with vDPA reset as well. One example is the Qemu.

quoted

As explained in previous threads [1][2], when IOTLB_PERSIST is not set
it doesn't necessarily mean the iotlb will definitely be destroyed
across reset (think about the platform IOMMU case), so userspace today
is already tolerating enough with either good or bad IOMMU.

I'm confused, how to define tolerating here? For example, if it has
tolerance, why bother?

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quoted

This code of

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not checking IOTLB_PERSIST being set is intentional, there's no point to
emulate bad IOMMU behavior even for older userspace (with improper
emulation to be done it would result in even worse performance).

I can easily imagine a case:

The old Qemu that works only with a setup like mlx5_vdpa. If we do
this without a negotiation, IOTLB will not be clear but the Qemu will
try to re-program the IOTLB after reset. Which will break?

1) stick the exact old behaviour with just one line of check
2) audit all the possible cases to avoid a one line of code

1) seems much easier than 2)

quoted

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For two reasons:

1) backend features need acked by userspace this is by design
2) keep the odd behaviour seems to be more safe as we can't audit
every userspace program

The old behavior (without flag ack) cannot be trusted already, as:

Possibly but the point is to unbreak userspace no matter how weird the
behaviour we've ever had.

quoted

* Devices using platform IOMMU (in other words, not implementing
neither .set_map nor .dma_map) does not unmap memory at virtio reset.
* Devices that implement .set_map or .dma_map (vdpa_sim, mlx5) do
reset IOTLB, but in their parent ops (vdpasim_do_reset, prune_iotlb
called from mlx5_vdpa_reset). With vdpa_sim patch removing the reset,
now all backends work the same as far as I know., which was (and is)
the way devices using the platform IOMMU works.

The difference in behavior did not matter as QEMU unmaps all the
memory unregistering the memory listener at vhost_vdpa_dev_start(...,
started = false),

Exactly. It's not just QEMU, but any (older) userspace manipulates
mappings through the vhost-vdpa iotlb interface has to unmap all
mappings to workaround the vdpa parent driver bug.

Just to clarify, from userspace, it's the (odd) behaviour of the current uAPI.

If they don't do
explicit unmap, it would cause state inconsistency between vhost-vdpa
and parent driver, then old mappings can't be restored, and new mapping
can be added to iotlb after vDPA reset. There's no point to preserve
this broken and inconsistent behavior between vhost-vdpa and parent
driver, as userspace doesn't care at all!

It's a userspace notice change so we can't fix it silently:

https://lkml.org/lkml/2012/12/23/75

Another example which is related to vhost-vDPA:

https://lore.kernel.org/netdev/20230927140544.205088-1-eric.auger@redhat.com/T/ (local)

Thanks

quoted

but the backend acknowledging this feature flag
allows QEMU to make sure it is safe to skip this unmap & map in the
case of vhost stop & start cycle.

In that sense, this feature flag is actually a signal for userspace to
know that the bug has been solved.

Right, I couldn't say it better than you do, thanks! The feature flag is
more of an unusual means to indicating kernel bug having been fixed,
rather than introduce a new feature or new kernel behavior ending up in
change of userspace's expectation.

quoted

Not offering it indicates that
userspace cannot trust the kernel will retain the maps.

Si-Wei or Dragos, please correct me if I've missed something. Feel
free to use the text in case you find more clear in doc or patch log.

Sure, will do, thank you! Will post v2 adding these to the log.

Thanks,
-Siwei

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Thanks!

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Thanks

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I think
the purpose of the IOTLB_PERSIST flag is just to give userspace 100%
certainty of persistent iotlb mapping not getting lost across vdpa reset.

Thanks,
-Siwei

[1]
https://lore.kernel.org/virtualization/9f118fc9-4f6f-dd67-a291-be78152e47fd@oracle.com/ (local)
[2]
https://lore.kernel.org/virtualization/3364adfd-1eb7-8bce-41f9-bfe5473f1f2e@oracle.com/ (local)

quoted

   Otherwise
we may break old userspace.

Thanks

quoted

+       vhost_vdpa_reset_map(v, asid);
          kfree(as);

          return 0;
--
1.8.3.1

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