linux-kernel - Re: [PATCH v2 0/5] VDUSE: Support registering userspace memory as bounce buffer

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Message-ID: <07a8e952-1347-8920-3a78-0200b865e0e9@redhat.com>
Date:   Thu, 14 Jul 2022 10:59:17 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Yongji Xie <xieyongji@...edance.com>
Cc:     mst <mst@...hat.com>, Liu Xiaodong <xiaodong.liu@...el.com>,
        Maxime Coquelin <maxime.coquelin@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        songmuchun@...edance.com,
        virtualization <virtualization@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/5] VDUSE: Support registering userspace memory as
 bounce buffer


在 2022/7/11 15:24, Yongji Xie 写道:
> On Mon, Jul 11, 2022 at 2:02 PM Jason Wang <jasowang@...hat.com> wrote:
>> On Fri, Jul 8, 2022 at 5:53 PM Yongji Xie <xieyongji@...edance.com> wrote:
>>> On Fri, Jul 8, 2022 at 4:38 PM Jason Wang <jasowang@...hat.com> wrote:
>>>> On Wed, Jul 6, 2022 at 6:16 PM Yongji Xie <xieyongji@...edance.com> wrote:
>>>>> On Wed, Jul 6, 2022 at 5:30 PM Jason Wang <jasowang@...hat.com> wrote:
>>>>>> On Wed, Jul 6, 2022 at 1:05 PM Xie Yongji <xieyongji@...edance.com> wrote:
>>>>>>> Hi all,
>>>>>>>
>>>>>>> This series introduces some new ioctls: VDUSE_IOTLB_GET_INFO,
>>>>>>> VDUSE_IOTLB_REG_UMEM and VDUSE_IOTLB_DEREG_UMEM to support
>>>>>>> registering and de-registering userspace memory for IOTLB
>>>>>>> as bounce buffer in virtio-vdpa case.
>>>>>>>
>>>>>>> The VDUSE_IOTLB_GET_INFO ioctl can help user to query IOLTB
>>>>>>> information such as bounce buffer size. Then user can use
>>>>>>> those information on VDUSE_IOTLB_REG_UMEM and
>>>>>>> VDUSE_IOTLB_DEREG_UMEM ioctls to register and de-register
>>>>>>> userspace memory for IOTLB.
>>>>>>>
>>>>>>> During registering and de-registering, the DMA data in use
>>>>>>> would be copied from kernel bounce pages to userspace bounce
>>>>>>> pages and back.
>>>>>>>
>>>>>>> With this feature, some existing application such as SPDK
>>>>>>> and DPDK can leverage the datapath of VDUSE directly and
>>>>>>> efficiently as discussed before [1][2]. They can register
>>>>>>> some preallocated hugepages to VDUSE to avoid an extra
>>>>>>> memcpy from bounce-buffer to hugepages.
>>>>>> This is really interesting.
>>>>>>
>>>>>> But a small concern on uAPI is that this seems to expose the VDUSE
>>>>>> internal implementation (bounce buffer) to userspace. We tried hard to
>>>>>> hide it via the GET_FD before. Anyway can we keep it?
>>>>>>
>>>>> Another way is changing GET_FD ioctl to add a flag or reuse 'perm'
>>>>> field to indicate whether a IOVA region supports userspace memory
>>>>> registration. Then userspace can use
>>>>> VDUSE_IOTLB_REG_UMEM/VDUSE_IOTLB_DEREG_UMEM to register/deregister
>>>>> userspace memory for this IOVA region.
>>>> Looks better.
>>>>
>>> OK.
>>>
>>>>> Any suggestions?
>>>> I wonder what's the value of keeping the compatibility with the kernel
>>>> mmaped bounce buffer. It means we need to take extra care on e.g data
>>>> copying when reg/reg user space memory.
>>>>
>>> I'm not sure I get your point on the compatibility with the kernel
>>> bounce buffer. Do you mean they use the same iova region?
>> Yes.
>>
>>> The userspace daemon might crash or reboot. In those cases, we still
>>> need a kernel buffer to store/recover the data.
>> Yes, this should be a good point.
>>
>>>> Can we simply allow the third kind of fd that only works for umem registration?
>>>>
>>> Do you mean using another iova region for umem?
>> I meant having a new kind of fd that only allows umem registration.
>>
> OK. It seems to be a little complicated to allow mapping a registered
> user memory via a new fd, e.g. how to handle the mapping if the
> userspace daemon exits but the fd is already passed to another
> process.
>
>>> I think we don't need
>>> a fd in umem case since the userspace daemon can access the memory
>>> directly without using mmap() to map it into the address space in
>>> advance.
>> Ok, I will have a look at the code and get back.
>>
> OK. Looking forward to your reply.


Looks good overall.

Just few comments.

Thanks


>
> Thanks,
> Yongji
>