Re: [dpdk-dev] [PATCH v3 01/19] Revert "vhost: workaround MQ fails to startup"

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 11/03/2017 02:05 PM, Yuanhan Liu wrote:
> On Thu, Nov 02, 2017 at 10:40:26AM +0100, Maxime Coquelin wrote:
>>> Moving from QEMU v2.7.0 to v2.10.0 resolves the issue. However, herein lies the issue: QEMU v2.10.0 was only released in August of this year; anecdotally, we know that many OvS-DPDK customers use older versions of QEMU (typically, v2.7.0), and are likely un[able|willing] to move. With this patch, a hard dependency on QEMU v2.10 is created for users who want to use the vHU multiq feature in DPDK v17.11 (and subsequently, the upcoming OvS v2.9.0), which IMO will likely be unacceptable for many.
>>
>> Do you mean that upstream Qemu v2.7.0 is used in production?
>> I would expect the customers to use a distro Qemu which should contain
>> relevant fixes, or follow upstream's stable branches.
>>
>> FYI, Qemu v2.9.1 contains a backport of the fix.
>>
>>> One potential solution to this problem is to introduce a compile-time option that would allow the user to [dis|en]able the VHOST_USER_PROTOCOL_F_REPLY_ACK feature - is that something that would be acceptable to you Maxime?
>>
>> Yes, that's one option, but:
>> 1. VHOST_USER_PROTOCOL_F_REPLY_ACK enabled should be the default
>> 2. VHOST_USER_PROTOCOL_F_REPLY_ACK disabled will be less extensively
>> tested.
>>
>> Yuanhan, what do you think?
> 
> My suggestion is to still disable it by default. Qemu 2.7 - 2.9 (inclusive)
> is a pretty big range, that I think quite many people would hit this issue
Ok, then what about adding a new flag to rte_vhost_driver_register(), as
done for tx zero copy to enable IOMMU feature?
If flag is unset, then we mask out both IOMMU virtio feature flag and
REPLY_ACK protocol feature flag.

For a while this flag will be unset by default, not to break these
deprecated and unmaintained Qemu versions. But I think at some point
we should make it enabled by default, as it would be sad not to benefit
from this security feature.

This change will have an impact on OVS, as it will need a new vhost-user
port option to enable IOMMU feature. Thing that is transparent to OVS
currently.

Mark, Yuanhan, does that sound good to you?

Maxime
> 	--yliu
>