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

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

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?

Besides, we really don't need multiple unmap/map for each vq. Would you 
think this shall be fixed in QEMU?

Thanks,
Jianfeng