From e06655cfe0fa9473b1e8b311571f36d787472834 Mon Sep 17 00:00:00 2001
From: Thomas Huth <thuth@redhat.com>
Date: Fri, 29 May 2020 05:54:02 -0400
Subject: [PATCH 20/42] vhost: correctly turn on VIRTIO_F_IOMMU_PLATFORM

RH-Author: Thomas Huth <thuth@redhat.com>
Message-id: <20200529055420.16855-21-thuth@redhat.com>
Patchwork-id: 97041
O-Subject: [RHEL-8.3.0 qemu-kvm PATCH v2 20/38] vhost: correctly turn on VIRTIO_F_IOMMU_PLATFORM
Bugzilla: 1828317
RH-Acked-by: Claudio Imbrenda <cimbrend@redhat.com>
RH-Acked-by: Cornelia Huck <cohuck@redhat.com>
RH-Acked-by: David Hildenbrand <david@redhat.com>

From: Jason Wang <jasowang@redhat.com>

We turn on device IOTLB via VIRTIO_F_IOMMU_PLATFORM unconditionally on
platform without IOMMU support. This can lead unnecessary IOTLB
transactions which will damage the performance.

Fixing this by check whether the device is backed by IOMMU and disable
device IOTLB.

Reported-by: Halil Pasic <pasic@linux.ibm.com>
Tested-by: Halil Pasic <pasic@linux.ibm.com>
Reviewed-by: Halil Pasic <pasic@linux.ibm.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20200302042454.24814-1-jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit f7ef7e6e3ba6e994e070cc609eb154339d1c4a11)
Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>
---
 hw/virtio/vhost.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index 9edfadc81d..9182a00495 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -290,7 +290,14 @@ static int vhost_dev_has_iommu(struct vhost_dev *dev)
 {
     VirtIODevice *vdev = dev->vdev;
 
-    return virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
+    /*
+     * For vhost, VIRTIO_F_IOMMU_PLATFORM means the backend support
+     * incremental memory mapping API via IOTLB API. For platform that
+     * does not have IOMMU, there's no need to enable this feature
+     * which may cause unnecessary IOTLB miss/update trnasactions.
+     */
+    return vdev->dma_as != &address_space_memory &&
+           virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
 }
 
 static void *vhost_memory_map(struct vhost_dev *dev, hwaddr addr,
@@ -765,6 +772,9 @@ static int vhost_dev_set_features(struct vhost_dev *dev,
     if (enable_log) {
         features |= 0x1ULL << VHOST_F_LOG_ALL;
     }
+    if (!vhost_dev_has_iommu(dev)) {
+        features &= ~(0x1ULL << VIRTIO_F_IOMMU_PLATFORM);
+    }
     r = dev->vhost_ops->vhost_set_features(dev, features);
     if (r < 0) {
         VHOST_OPS_DEBUG("vhost_set_features failed");
-- 
2.27.0

