[RFC] iommu/virtio: Use single flush queue (EXPERIMENTAL)

[Niklas Schnelle]

Just like on paged s390 guests with their virtual IOMMU, syncing
mappings via virtio-iommu is quite expensive. It can thus benefit from
queueing unmapped IOVAs and flushing them in batches but less so from
parallel flushes which is what the shadow_on_flush flag introduced for
s390 tunes dma-iommu to do.

For this to work .flush_iotlb_all is implemented. Furthermore
.iotlb_sync_map is also implemented and used to pull the sync out of the
mapping operation for some additional batching and performance gain.

In a basic test with NVMe pass-through to a KVM guest on a Ryzen 3900X
these changes together lead to about 19% more IOPS in a fio test and
slightly more bandwidth too.

Signed-off-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx>
---
Note:
The idea of using the single flush queue scheme from my series "iommu/dma: s390
DMA API conversion and optimized IOTLB flushing"[0] for virtio-iommu was already
mentioned in the cover letter. I now wanted to explore this with this patch
which may also serve as a test vehicle for the single flush queue scheme usable
on non-s390.

Besides limited testing, this is marked experimental mainly because the use of
queuing needs to be a concious decision as it allows continued access to
unmapped pages for up to a second with the currently proposed single flush
queue mechanism. Also it might make sense to split this patch to do the
introduction and use of .iotlb_sync_map separately but as a test vehicle
I found it easier to consume as a single patch.

[0]: https://lore.kernel.org/linux-iommu/20230717-dma_iommu-v11-0-a7a0b83c355c@xxxxxxxxxxxxx/

 drivers/iommu/virtio-iommu.c | 28 +++++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/virtio-iommu.c b/drivers/iommu/virtio-iommu.c
index 3551ed057774..f29eb4ce2b88 100644
--- a/drivers/iommu/virtio-iommu.c
+++ b/drivers/iommu/virtio-iommu.c
@@ -843,7 +843,7 @@ static int viommu_map_pages(struct iommu_domain *domain, unsigned long iova,
 			.flags		= cpu_to_le32(flags),
 		};
 
-		ret = viommu_send_req_sync(vdomain->viommu, &map, sizeof(map));
+		ret = viommu_add_req(vdomain->viommu, &map, sizeof(map));
 		if (ret) {
 			viommu_del_mappings(vdomain, iova, end);
 			return ret;
@@ -909,6 +909,27 @@ static void viommu_iotlb_sync(struct iommu_domain *domain,
 {
 	struct viommu_domain *vdomain = to_viommu_domain(domain);
 
+	if (!vdomain->nr_endpoints)
+		return;
+	viommu_sync_req(vdomain->viommu);
+}
+
+static int viommu_iotlb_sync_map(struct iommu_domain *domain,
+				 unsigned long iova, size_t size)
+{
+	struct viommu_domain *vdomain = to_viommu_domain(domain);
+
+	if (!vdomain->nr_endpoints)
+		return 0;
+	return viommu_sync_req(vdomain->viommu);
+}
+
+static void viommu_flush_iotlb_all(struct iommu_domain *domain)
+{
+	struct viommu_domain *vdomain = to_viommu_domain(domain);
+
+	if (!vdomain->nr_endpoints)
+		return;
 	viommu_sync_req(vdomain->viommu);
 }
 
@@ -991,6 +1012,7 @@ static struct iommu_device *viommu_probe_device(struct device *dev)
 		if (ret)
 			goto err_free_dev;
 	}
+	dev->iommu->shadow_on_flush = 1;
 
 	return &viommu->iommu;
 
@@ -1037,6 +1059,8 @@ static bool viommu_capable(struct device *dev, enum iommu_cap cap)
 	switch (cap) {
 	case IOMMU_CAP_CACHE_COHERENCY:
 		return true;
+	case IOMMU_CAP_DEFERRED_FLUSH:
+		return true;
 	default:
 		return false;
 	}
@@ -1057,7 +1081,9 @@ static struct iommu_ops viommu_ops = {
 		.map_pages		= viommu_map_pages,
 		.unmap_pages		= viommu_unmap_pages,
 		.iova_to_phys		= viommu_iova_to_phys,
+		.flush_iotlb_all	= viommu_flush_iotlb_all,
 		.iotlb_sync		= viommu_iotlb_sync,
+		.iotlb_sync_map		= viommu_iotlb_sync_map,
 		.free			= viommu_domain_free,
 	}
 };

base-commit: 5514392fe77cd45b0d33bf239f13ba594a6759e5
-- 
2.39.2