On Tue, Nov 5, 2024 at 2:53 PM Xuan Zhuo [off-list ref] wrote:

On Tue, 5 Nov 2024 11:42:09 +0800, Jason Wang [off-list ref] wrote:

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On Wed, Oct 30, 2024 at 4:25 PM Xuan Zhuo [off-list ref] wrote:

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The subsequent commit needs to know whether every indirect buffer is
premapped or not. So we need to introduce an extra struct for every
indirect buffer to record this info.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
---
 drivers/virtio/virtio_ring.c | 112 ++++++++++++++++-------------------
 1 file changed, 52 insertions(+), 60 deletions(-)

Do we have a performance impact for this patch?

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diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 97590c201aa2..dca093744fe1 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c

@@ -69,7 +69,11 @@

 struct vring_desc_state_split {
        void *data;                     /* Data for callback. */
-       struct vring_desc *indir_desc;  /* Indirect descriptor, if any. */
+
+       /* Indirect extra table and desc table, if any. These two will be
+        * allocated together. So we won't stress more to the memory allocator.
+        */
+       struct vring_desc *indir_desc;

So it looks like we put a descriptor table after the extra table. Can
this lead to more crossing page mappings for the indirect descriptors?

If yes, it seems expensive so we probably need to make the descriptor
table come first.

No, the descriptors are before extra table.

Well, you need then tweak the above comment, it said

"Indirect extra table and desc table".

So, there is not performance impact.

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 };

[...]

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        while (vq->split.vring.desc[i].flags & nextflag) {
-               vring_unmap_one_split(vq, i);
+               vring_unmap_one_split(vq, &extra[i]);

Not sure if I've asked this before. But this part seems to deserve an
independent fix for -stable.

What fix?

I meant for hardening we need to check the flags stored in the extra
instead of the descriptor itself as it could be mangled by the device.

Thanks