On 4/24/2024 12:33 AM, Eugenio Perez Martin wrote:
On Wed, Apr 24, 2024 at 12:21 AM Si-Wei Liu <si-wei.liu@oracle.com> wrote:
On 4/22/2024 1:49 AM, Eugenio Perez Martin wrote:
On Sat, Apr 20, 2024 at 1:50 AM Si-Wei Liu <si-wei.liu@oracle.com> wrote:
On 4/19/2024 1:29 AM, Eugenio Perez Martin wrote:
On Thu, Apr 18, 2024 at 10:46 PM Si-Wei Liu <si-wei.liu@oracle.com> wrote:
On 4/10/2024 3:03 AM, Eugenio Pérez wrote:
IOVA tree is also used to track the mappings of virtio-net shadow
virtqueue. This mappings may not match with the GPA->HVA ones.
This causes a problem when overlapped regions (different GPA but same
translated HVA) exists in the tree, as looking them by HVA will return
them twice. To solve this, create an id member so we can assign unique
identifiers (GPA) to the maps.
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
include/qemu/iova-tree.h | 5 +++--
util/iova-tree.c | 3 ++-
2 files changed, 5 insertions(+), 3 deletions(-)
diff --git a/include/qemu/iova-tree.h b/include/qemu/iova-tree.h
index 2a10a7052e..34ee230e7d 100644
--- a/include/qemu/iova-tree.h
+++ b/include/qemu/iova-tree.h
@@ -36,6 +36,7 @@ typedef struct DMAMap {
hwaddr iova;
hwaddr translated_addr;
hwaddr size; /* Inclusive */
+ uint64_t id;
IOMMUAccessFlags perm;
} QEMU_PACKED DMAMap;
typedef gboolean (*iova_tree_iterator)(DMAMap *map);
@@ -100,8 +101,8 @@ const DMAMap *iova_tree_find(const IOVATree *tree, const
DMAMap *map);
* @map: the mapping to search
*
* Search for a mapping in the iova tree that translated_addr overlaps
with the
- * mapping range specified. Only the first found mapping will be
- * returned.
+ * mapping range specified and map->id is equal. Only the first found
+ * mapping will be returned.
*
* Return: DMAMap pointer if found, or NULL if not found. Note that
* the returned DMAMap pointer is maintained internally. User should
diff --git a/util/iova-tree.c b/util/iova-tree.c
index 536789797e..0863e0a3b8 100644
--- a/util/iova-tree.c
+++ b/util/iova-tree.c
@@ -97,7 +97,8 @@ static gboolean iova_tree_find_address_iterator(gpointer key,
gpointer value,
needle = args->needle;
if (map->translated_addr + map->size < needle->translated_addr ||
- needle->translated_addr + needle->size < map->translated_addr) {
+ needle->translated_addr + needle->size < map->translated_addr ||
+ needle->id != map->id) {
It looks this iterator can also be invoked by SVQ from
vhost_svq_translate_addr() -> iova_tree_find_iova(), where guest GPA
space will be searched on without passing in the ID (GPA), and exact
match for the same GPA range is not actually needed unlike the mapping
removal case. Could we create an API variant, for the SVQ lookup case
specifically? Or alternatively, add a special flag, say skip_id_match to
DMAMap, and the id match check may look like below:
(!needle->skip_id_match && needle->id != map->id)
I think vhost_svq_translate_addr() could just call the API variant or
pass DMAmap with skip_id_match set to true to svq_iova_tree_find_iova().
I think you're totally right. But I'd really like to not complicate
the API of the iova_tree more.
I think we can look for the hwaddr using memory_region_from_host and
then get the hwaddr. It is another lookup though...
Yeah, that will be another means of doing translation without having to
complicate the API around iova_tree. I wonder how the lookup through
memory_region_from_host() may perform compared to the iova tree one, the
former looks to be an O(N) linear search on a linked list while the
latter would be roughly O(log N) on an AVL tree?
Even worse, as the reverse lookup (from QEMU vaddr to SVQ IOVA) is
linear too. It is not even ordered.
Oh Sorry, I misread the code and I should look for g_tree_foreach ()
instead of g_tree_search_node(). So the former is indeed linear
iteration, but it looks to be ordered?
https://urldefense.com/v3/__https://github.com/GNOME/glib/blob/main/glib/gtree.c*L1115__;Iw!!ACWV5N9M2RV99hQ!Ng2rLfRd9tLyNTNocW50Mf5AcxSt0uF0wOdv120djff-z_iAdbujYK-jMi5UC1DZLxb1yLUv2vV0j3wJo8o$
The GPA / IOVA are ordered but we're looking by QEMU's vaddr.
If we have these translations:
[0x1000, 0x2000] -> [0x10000, 0x11000]
[0x2000, 0x3000] -> [0x6000, 0x7000]
We will see them in this order, so we cannot stop the search at the first node.
Yeah, reverse lookup is unordered indeed, anyway.
But apart from this detail you're right, I have the same concerns with
this solution too. If we see a hard performance regression we could go
to more complicated solutions, like maintaining a reverse IOVATree in
vhost-iova-tree too. First RFCs of SVQ did that actually.
Agreed, yeap we can use memory_region_from_host for now. Any reason why
reverse IOVATree was dropped, lack of users? But now we have one!
No, it is just simplicity. We already have an user in the hot patch in
the master branch, vhost_svq_vring_write_descs. But I never profiled
enough to find if it is a bottleneck or not to be honest.
Right, without vIOMMU or a lot of virtqueues / mappings, it's hard to
profile and see the difference.
I'll send the new series by today, thank you for finding these issues!
Thanks! In case you don't have bandwidth to add back reverse IOVA tree,
Jonah (cc'ed) may have interest in looking into it.