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Re: [PATCH] vhost: Unbreak SMMU and virtio-iommu on dev-iotlb support

From: Jason Wang
Subject: Re: [PATCH] vhost: Unbreak SMMU and virtio-iommu on dev-iotlb support
Date: Fri, 5 Feb 2021 11:16:39 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 2021/2/5 上午3:12, Peter Xu wrote:
Previous work on dev-iotlb message broke vhost on either SMMU

Have a quick git grep and it looks to me v3 support ATS and have command for device iotlb (ATC) invalidation.

or virtio-iommu
since dev-iotlb (or PCIe ATS)

We may need to add this in the future.

is not yet supported for those archs.

Rethink about this, it looks to me the point is that we should make vhost work when ATS is disabled like what ATS spec defined:


ATS is enabled through a new Capability and associated configuration structure.  To enable 15 ATS, software must detect this Capability and enable the Function to issue ATS TLP.  If a Function is not enabled, the Function is required not to issue ATS Translation Requests and is required to issue all DMA Read and Write Requests with the TLP AT field set to “untranslated.”


Maybe we can add this in the commit log.

An initial idea is that we can let IOMMU to export this information to vhost so
that vhost would know whether the vIOMMU would support dev-iotlb, then vhost
can conditionally register to dev-iotlb or the old iotlb way.  We can work
based on some previous patch to introduce PCIIOMMUOps as Yi Liu proposed [1].

However it's not as easy as I thought since vhost_iommu_region_add() does not
have a PCIDevice context at all since it's completely a backend.  It seems
non-trivial to pass over a PCI device to the backend during init.  E.g. when
the IOMMU notifier registered hdev->vdev is still NULL.

To make the fix smaller and easier, this patch goes the other way to leverage
the flag_changed() hook of vIOMMUs so that SMMU and virtio-iommu can trap the
dev-iotlb registration and fail it.  Then vhost could try the fallback solution
as using UNMAP invalidation for it's translations.


Reported-by: Eric Auger <eric.auger@redhat.com>
Fixes: b68ba1ca57677acf870d5ab10579e6105c1f5338
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
  hw/arm/smmuv3.c          |  5 +++++
  hw/virtio/vhost.c        | 13 +++++++++++--
  hw/virtio/virtio-iommu.c |  5 +++++
  3 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/hw/arm/smmuv3.c b/hw/arm/smmuv3.c
index 98b99d4fe8e..bd1f97000d9 100644
--- a/hw/arm/smmuv3.c
+++ b/hw/arm/smmuv3.c
@@ -1497,6 +1497,11 @@ static int smmuv3_notify_flag_changed(IOMMUMemoryRegion 
      SMMUv3State *s3 = sdev->smmu;
      SMMUState *s = &(s3->smmu_state);
+        error_setg(errp, "SMMUv3 does not support dev-iotlb yet");
+        return -EINVAL;
+    }
      if (new & IOMMU_NOTIFIER_MAP) {
                     "device %02x.%02x.%x requires iommu MAP notifier which is "
diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index 28c7d781721..6e17d631f77 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -704,6 +704,7 @@ static void vhost_iommu_region_add(MemoryListener *listener,
      Int128 end;
      int iommu_idx;
      IOMMUMemoryRegion *iommu_mr;
+    int ret;
if (!memory_region_is_iommu(section->mr)) {
@@ -726,8 +727,16 @@ static void vhost_iommu_region_add(MemoryListener 
      iommu->iommu_offset = section->offset_within_address_space -
      iommu->hdev = dev;
-    memory_region_register_iommu_notifier(section->mr, &iommu->n,
-                                          &error_fatal);
+    ret = memory_region_register_iommu_notifier(section->mr, &iommu->n, NULL);
+    if (ret) {
+        /*
+         * Some vIOMMUs do not support dev-iotlb yet.  If so, try to use the
+         * UNMAP legacy message
+         */
+        iommu->n.notifier_flags = IOMMU_NOTIFIER_UNMAP;
+        memory_region_register_iommu_notifier(section->mr, &iommu->n,
+                                              &error_fatal);
+    }
      QLIST_INSERT_HEAD(&dev->iommu_list, iommu, iommu_next);
      /* TODO: can replay help performance here? */
diff --git a/hw/virtio/virtio-iommu.c b/hw/virtio/virtio-iommu.c
index 6b9ef7f6b2b..c2883a2f6c8 100644
--- a/hw/virtio/virtio-iommu.c
+++ b/hw/virtio/virtio-iommu.c
@@ -893,6 +893,11 @@ static int 
virtio_iommu_notify_flag_changed(IOMMUMemoryRegion *iommu_mr,
                                              IOMMUNotifierFlag new,
                                              Error **errp)
+        error_setg(errp, "Virtio-iommu does not support dev-iotlb yet");
+        return -EINVAL;
+    }
      if (old == IOMMU_NOTIFIER_NONE) {
      } else if (new == IOMMU_NOTIFIER_NONE) {

Patch looks good. I wonder whether we should fix intel when ATS is disabled.


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