Re: [dpdk-dev] [PATCH v2 5/7] net/virtio_user: add vhost kernel support

[Jason Wang <jasowang@redhat.com>]

On 2017年01月04日 15:22, Tan, Jianfeng wrote:
>
> Sorry, I forget to reply this comment.
>
> On 12/26/2016 3:44 PM, Yuanhan Liu wrote:
>> [...]
>>> +
>>> +    /* Does not work when VIRTIO_F_IOMMU_PLATFORM now, why? */
>> Because this feature need the vhost IOTLB support from the device
>> emulation. Patches for QEMU hasn't been merged yet, but it has been
>> there for a while.
>
> Yes. And it's in need of help from QEMU.
>
>>
>> Since we don't have the support yet, for sure it won't work. But
>> I'm wondering why you have to disable it explicitly?
>
> Here we do not have QEMU. Frontend driver talks with vhost-net through 
> virtio_user_dev. And both ends claim to support 
> VIRTIO_F_IOMMU_PLATFORM. So this feature bit will be negotiated if we 
> don't explicitly disable it. In my previous test, it fails in 
> vhost_init_device_iotlb() of vhost kernel module.

Interesting, vhost_init_device_iotlb() only fail when OOM. Do you meet it?


> My guess is that, for this feature, without the help of QEMU, vhost 
> kernel module cannot work independently.

Technically it can if your userspace supports device IOTLB APIs too.

Or if you're using static mappings, and preset all mappings through 
VHOST_IOTLB_UPADATE to make sure no IOTLB misses.

Thanks

>
> Thanks,
> Jianfeng