Re: [dpdk-dev] [PATCH v2 2/3] vhost: protect dirty logging against logging base change

["Tan, Jianfeng" <jianfeng.tan@intel.com>]

> -----Original Message-----
> From: Maxime Coquelin [mailto:maxime.coquelin@redhat.com]
> Sent: Wednesday, February 14, 2018 3:53 PM
> To: Tan, Jianfeng; dev@dpdk.org; yliu@fridaylinux.org; Bie, Tiwei;
> vkaplans@redhat.com
> Cc: stable@dpdk.org; jfreiman@redhat.com
> Subject: Re: [PATCH v2 2/3] vhost: protect dirty logging against logging base
> change
> 
> Hi Jianfeng,
> 
> On 02/14/2018 03:03 AM, Tan, Jianfeng wrote:
> > Hi Maxime,
> >
> >
> > On 11/28/2017 6:06 PM, Maxime Coquelin wrote:
> >>
> >>
> >> On 11/24/2017 07:08 PM, Maxime Coquelin wrote:
> >>> When performing live-migration with multiple queue pairs,
> >>> VHOST_USER_SET_LOG_BASE request is sent multiple times.
> >>>
> >>> If packets are being processed by the PMD threads, it is
> >>> possible that they are setting bits in the dirty log map while
> >>> its region is being unmapped by the vhost-user protocol thread.
> >>> It results in the following crash:
> >>> Thread 3 "lcore-slave-2" received signal SIGSEGV, Segmentation fault.
> >>> [Switching to Thread 0x7f71ca495700 (LWP 32451)]
> >>> 0x00000000004bfc8a in vhost_set_bit (addr=0x7f71cbe18432 <error:
> >>> Cannot access memory at address 0x7f71cbe18432>, nr=1) at
> >>> /home/max/projects/src/mainline/dpdk/lib/librte_vhost/vhost.h:267
> >>> 267        __sync_fetch_and_or_8(addr, (1U << nr));
> >>>
> >>> We can see the vhost-user protocol thread just did the unmap of the
> >>> dirty log region when it happens.
> >>>
> >>> This patch prevents this by introducing a RW lock to protect
> >>> the log base.
> >>>
> >>> Fixes: 54f9e32305d4 ("vhost: handle dirty pages logging request")
> >>> Cc: stable@dpdk.org
> >>>
> >>> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> >>> ---
> >>>   lib/librte_vhost/vhost.c      |  2 ++
> >>>   lib/librte_vhost/vhost.h      | 14 +++++++++++---
> >>>   lib/librte_vhost/vhost_user.c |  4 ++++
> >>>   3 files changed, 17 insertions(+), 3 deletions(-)
> >>>
> >>
> >> By clarifying the vhost-user spec, we may be able to avoid this lock and
> >> just ignore the subsequent SET_LOG_BASE requests once
> >> VHOST_F_LOG_ALL feature bit is set.
> >>
> >> So let's just discard this series for now.
> >
> > I would assume this issue has been addressed by the per-queue lock patch
> > from Victor, correct?
> 
> Correct.
> 
> > Besides, we really don't need multiple unmap/map for each vq. Would you
> > think this shall be fixed in QEMU?
> 
> Yes, I tihnk you are right it should be fixed in QEMU, so that it is
> sent only for the first queue pair.
> 
> But I didn't had time to work on it TBH.

Thank you for the confirmation. And it's not an urgent issue to fix anyway.