From: Maxime Coquelin <maxime.coquelin@redhat.com>
To: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Cc: "Zhiyong Yang" <zhiyong.yang@intel.com>,
dev@dpdk.org, ciara.loftus@intel.com,
"Marc-André Lureau" <mlureau@redhat.com>,
"Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [dpdk-dev] [PATCH] vhost: fix MQ fails to startup
Date: Fri, 28 Apr 2017 09:23:54 +0200 [thread overview]
Message-ID: <9ecbbd0b-a2a0-7674-e84d-c8beeeeff0e6@redhat.com> (raw)
In-Reply-To: <20170428022558.GM11512@yliu-dev.sh.intel.com>
On 04/28/2017 04:25 AM, Yuanhan Liu wrote:
> On Thu, Apr 27, 2017 at 10:52:20AM +0200, Maxime Coquelin wrote:
>>
>>
>> On 04/27/2017 10:20 AM, Yuanhan Liu wrote:
>>> On Thu, Apr 27, 2017 at 09:56:47AM +0200, Maxime Coquelin wrote:
>>>> Hi Zhiyong,
>>>>
>>>> +Marc-André
>>>>
>>>> On 04/27/2017 08:34 AM, Zhiyong Yang wrote:
>>>>> vhost since dpdk17.02 + qemu2.7 and above will cause failures of
>>>>> new connection when negotiating to set MQ. (one queue pair works
>>>>> well).Because there exist some bugs in qemu code when introducing
>>>>> VHOST_USER_PROTOCOL_F_REPLY_ACK to qemu. when dealing with the vhost
>>>>> message VHOST_USER_SET_MEM_TABLE for the second time, qemu indeed
>>>>> doesn't send the messge (The message needs to be sent only once)but
>>>>> still will be waiting for dpdk's reply ack, then, qemu is always
>>>>> freezing. DPDK code works in the right way.
>>>>
>>>> I'm looking at Qemu's vhost_user_set_mem_table() function, but fail to
>>>> see how it could wait for the reply-ack if it didn't send the
>>>> VHOST_USER_SET_MEM_TABLE request before.
>>>>
>>>>> But the feature
>>>>> VHOST_USER_PROTOCOL_F_REPLY_ACK has to be disabled by default at the
>>>>> dpdk side in order to avoid the feature support of DPDK + qemu at
>>>>> the same time. if doing like that, MQ can works well. Once Qemu bugs
>>>>> have been fixed and upstreamed, we can enable it.
>>>>
>>>> The problem is for DPDK to detect whether bug is fixed in Qemu.
>>>> Maybe only way would be to have a new protocol feature flag, which is
>>>> not really its role.
>>>
>>> Wouldn't that be an overkill, judging that REPLY_ACK is not a must
>>> feature?
>>
>> Yes, maybe. But it was introduced to fix (possible) race conditions:
>> https://lists.gnu.org/archive/html/qemu-devel/2016-07/msg06173.html
>
> But AFAIK, that commit has been reverted:
>
> commit 94c9cb31c04737f86be29afefbff401cd23bc24d
> Author: Michael S. Tsirkin <mst@redhat.com>
> Date: Mon Aug 15 16:35:24 2016 +0300
>
> Revert "vhost-user: Attempt to fix a race with set_mem_table."
>
> This reverts commit 28ed5ef16384f12500abd3647973ee21b03cbe23.
>
> I still think it's the right thing to do, but
> tests have been failing sporadically.
>
> Revert for now, and hope to fix it before the release.
No, what has been reverted is a workaround when REPLY_ACK protocol
feature has not been negotiated.
Instead of waiting for the backend to send the ack, the workaround
consisted in sending a GET_FEATURES request after having sent the
SET_MEM_TABLE request, in order to ensure SET_MEM_TABLE request handling
was done before.
The problem is that it sometimes created a deadlock when when running
QEMU's vhost-user-test in TCG mode.
>>
>> Note that I planned to use this feature for the device IOTLB
>> implementation to let the backend decide whether it wants the IOTLB
>> misses synchronous or asynchronous. But I can still change the protocol
>> spec to make this behavior specific to this request.
>
> Maybe we could introduce a version message? With that, we could tell
> whether the frontend has fixed the known bug or not.
That's a possibility, but this is not really the role of a protocol
version. As in this case, the protocol does not change, just an
implementation.
> Note that we already has the "version" info in current vhost-user spec.
> It's just 2 bits in the message "flag" field though, which is not quite
> enough.
Indeed, it does not let room for lots of bugs :)
Thanks,
Maxime
> --yliu
>
next prev parent reply other threads:[~2017-04-28 7:24 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-27 6:34 Zhiyong Yang
2017-04-27 7:41 ` Loftus, Ciara
2017-04-27 7:56 ` Maxime Coquelin
2017-04-27 8:05 ` Maxime Coquelin
2017-04-27 8:24 ` Yang, Zhiyong
2017-04-27 8:32 ` Maxime Coquelin
2017-04-27 8:20 ` Yuanhan Liu
2017-04-27 8:52 ` Maxime Coquelin
2017-04-28 2:25 ` Yuanhan Liu
2017-04-28 7:23 ` Maxime Coquelin [this message]
2017-04-28 7:35 ` Yuanhan Liu
2017-04-28 7:39 ` Yuanhan Liu
2017-04-28 7:57 ` Maxime Coquelin
2017-04-28 8:00 ` Yuanhan Liu
2017-04-27 8:12 ` Yuanhan Liu
2017-04-27 8:32 ` Yang, Zhiyong
2017-04-27 9:41 ` [dpdk-dev] [PATCH v2] vhost: workaround " Zhiyong Yang
2017-04-27 10:00 ` Maxime Coquelin
2017-04-28 4:29 ` Yuanhan Liu
2017-05-10 2:07 ` Yang, Zhiyong
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