On Fri, 27 Jan 2017 13:24:13 +0100
Halil Pasic [off-list ref] wrote:

On 01/26/2017 08:20 PM, Michael S. Tsirkin wrote:

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On Thu, Jan 26, 2017 at 06:39:14PM +0100, Halil Pasic wrote:  

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Hi!

Recently I have been investigating some strange migration problems on
s390x.

It turned out under certain circumstances vhost_net corrupts avail.idx by
using wrong endianness.  

[..]

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-------------------------8<--------------  

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From b26e2bbdc03832a0204ee2b42967a1b49a277dc8 Mon Sep 17 00:00:00 2001  

From: Halil Pasic <redacted>
Date: Thu, 26 Jan 2017 00:06:15 +0100
Subject: [PATCH] vhost: remove useless/dangerous reset of is_le

The reset of is_le does no good, but it contributes its fair share to a
bug in vhost_net, which occurs if we have some oldubufs when stopping and
setting a fd = -1 as a backend. Instead of doing something convoluted in
vhost_net, let's just get rid of the reset.

Signed-off-by: Halil Pasic <redacted>
Fixes: commit 2751c9882b94 
---
 drivers/vhost/vhost.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index d643260..08072a2 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c

@@ -1714,10 +1714,8 @@ int vhost_vq_init_access(struct vhost_virtqueue *vq)
        int r;
        bool is_le = vq->is_le;

-       if (!vq->private_data) {
-               vhost_reset_is_le(vq);
+       if (!vq->private_data)
                return 0;
-       }

        vhost_init_is_le(vq);  

I think you do need to reset it, just maybe within vhost_init_is_le.

        if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
                vq->is_le = true;
	else
		vhost_reset_is_le(vq);

  

That is a very good point! I have overlooked that while the 
CONFIG_VHOST_CROSS_ENDIAN_LEGACY variant

static void vhost_init_is_le(struct vhost_virtqueue *vq)
{
        /* Note for legacy virtio: user_be is initialized at reset time
         * according to the host endianness. If userspace does not set an
         * explicit endianness, the default behavior is native endian, as
         * expected by legacy virtio.
         */
        vq->is_le = vhost_has_feature(vq, VIRTIO_F_VERSION_1) || !vq->user_be;
}

is fine the other variant 

static void vhost_init_is_le(struct vhost_virtqueue *vq)
{
        if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
                vq->is_le = true;
}
is a very strange initializer (makes assumptions about the state
to be initialized).

I agree, setting native endianness there sounds very reasonable.

I have a question regarding readability. IMHO the relationship
of reset_is_le and int_is_le is a bit confusing, and I'm afraid
it could become even more confusing with using reset in one of
the init_is_le's.

How about we do the following?

static void vhost_init_is_le(struct vhost_virtqueue *vq)
{
        if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
                vq->is_le = true;
+       else
+               vq->is_le = virtio_legacy_is_little_endian();

}

static void vhost_reset_is_le(struct vhost_virtqueue *vq)
{
-       vq->is_le = virtio_legacy_is_little_endian();
+       vhost_init_is_le(vq);
}

That way we would have correct endianness both after reset
and after init, I think :).

Yes, I think this is what we need.

Cheers.

--
Greg

Thank you very much!

Halil