[PATCH] virtio_ring: Fix the stale index in available ring

[Keir Fraser]

On Wed, Mar 20, 2024 at 03:24:16PM +1000, Gavin Shan wrote:
> 
> Before this patch was posted, I had debugging code to record last 16 transactions
> to the available and used queue from guest and host side. It did reveal the wrong
> head was fetched from the available queue.
> 
> [   11.785745] ================ virtqueue_get_buf_ctx_split ================
> [   11.786238] virtio_net virtio0: output.0:id 74 is not a head!
> [   11.786655] head to be released: 036 077
> [   11.786952]
> [   11.786952] avail_idx:
> [   11.787234] 000  63985  <--
> [   11.787237] 001  63986
> [   11.787444] 002  63987
> [   11.787632] 003  63988
> [   11.787821] 004  63989
> [   11.788006] 005  63990
> [   11.788194] 006  63991
> [   11.788381] 007  63992
> [   11.788567] 008  63993
> [   11.788772] 009  63994
> [   11.788957] 010  63995
> [   11.789141] 011  63996
> [   11.789327] 012  63997
> [   11.789515] 013  63998
> [   11.789701] 014  63999
> [   11.789886] 015  64000

Does the error always occur at such a round idx value?

Here, 64000 == 0xFA00. Maybe coincidence but it's improbable enough to be interesting.

This debug code seems rather useful!

 -- Keir



> [   11.790068]
> [   11.790068] avail_head:
> [   11.790529] 000  075  <--
> [   11.790718] 001  036
> [   11.790890] 002  077
> [   11.791061] 003  129
> [   11.791231] 004  072
> [   11.791400] 005  130
> [   11.791574] 006  015
> [   11.791748] 007  074
> [   11.791918] 008  130
> [   11.792094] 009  130
> [   11.792263] 010  074
> [   11.792437] 011  015
> [   11.792617] 012  072
> [   11.792788] 013  129
> [   11.792961] 014  077    // The last two heads from guest to host: 077, 036
> [   11.793134] 015  036
> 
> [root at nvidia-grace-hopper-05 qemu.main]# cat /proc/vhost
> 
> avail_idx
> 000  63998
> 001  64000
> 002  63954  <---
> 003  63955
> 004  63956
> 005  63974
> 006  63981
> 007  63984
> 008  63986
> 009  63987
> 010  63988
> 011  63989
> 012  63992
> 013  63993
> 014  63995
> 015  63997
> 
> avail_head
> 000  074
> 001  015
> 002  072
> 003  129
> 004  074            // The last two heads seen by vhost is: 074, 036
> 005  036
> 006  075  <---
> 007  036
> 008  077
> 009  129
> 010  072
> 011  130
> 012  015
> 013  074
> 014  130
> 015  130
> 
> used_idx
> 000  64000
> 001  63882  <---
> 002  63889
> 003  63891
> 004  63898
> 005  63936
> 006  63942
> 007  63946
> 008  63949
> 009  63953
> 010  63957
> 011  63981
> 012  63990
> 013  63992
> 014  63993
> 015  63999
> 
> used_head
> 000  072
> 001  129
> 002  074          // The last two heads published to guest is: 074, 036
> 003  036
> 004  075  <---
> 005  036
> 006  077
> 007  129
> 008  072
> 009  130
> 010  015
> 011  074
> 012  130
> 013  130
> 014  074
> 015  015
> 
> Thanks,
> Gavin
> 
> 
> 
>