Re: [PATCH RFC v2 03/10] vringh: reset kiov 'consumed' field in __vringh_iov()

From: Jason Wang
Date: Mon Feb 01 2021 - 22:28:15 EST



On 2021/2/1 下午6:21, Stefano Garzarella wrote:
On Mon, Feb 01, 2021 at 01:40:01PM +0800, Jason Wang wrote:

On 2021/1/28 下午10:41, Stefano Garzarella wrote:
__vringh_iov() overwrites the contents of riov and wiov, in fact it
resets the 'i' and 'used' fields, but also the consumed field should
be reset to avoid an inconsistent state.

Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>


I had a question(I remember we had some discussion like this but I forget the conclusion):

Sorry, I forgot to update you.


I see e.g in vringh_getdesc_kern() it has the following comment:

/*
 * Note that you may need to clean up riov and wiov, even on error!
 */

So it looks to me the correct way is to call vringh_kiov_cleanup() before?

Looking at the code the right pattern should be:

    vringh_getdesc_*(..., &out_iov, &in_iov, ...);

    // use out_iov and in_iov

    vringh_kiov_cleanup(&out_iov);
    vringh_kiov_cleanup(&in_iov);

This because vringh_getdesc_*() calls __vringh_iov() where resize_iovec() is called to allocate the iov wrapped by 'struct vringh_kiov' and vringh_kiov_cleanup() frees that memory.

Looking better, __vringh_iov() is able to extend a 'vringh_kiov' pre-allocated, so in order to avoid to allocate and free the iov for each request we can avoid to call vringh_kiov_cleanup(), but this patch is needed to avoid an inconsistent state.

And also patch "vdpa_sim: cleanup kiovs in vdpasim_free()" is required to free the iov when the device is going away.

Does that make sense to you?


Make sense.



Maybe I should add a comment in vringh.c to explain this better.


Yes, please.

Thanks



Thanks,
Stefano


Thanks


---
 drivers/vhost/vringh.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
index f68122705719..bee63d68201a 100644
--- a/drivers/vhost/vringh.c
+++ b/drivers/vhost/vringh.c
@@ -290,9 +290,9 @@ __vringh_iov(struct vringh *vrh, u16 i,
         return -EINVAL;
     if (riov)
-        riov->i = riov->used = 0;
+        riov->i = riov->used = riov->consumed = 0;
     if (wiov)
-        wiov->i = wiov->used = 0;
+        wiov->i = wiov->used = wiov->consumed = 0;
     for (;;) {
         void *addr;