linux-kernel - Re: [RFC PATCH v3 05/12] netdev: netdevice devmem allocator

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Message-ID: <a5b95e6b-8716-4e2e-9183-959b754b5b5e@kernel.org>
Date:   Tue, 7 Nov 2023 15:55:22 -0700
From:   David Ahern <dsahern@...nel.org>
To:     Mina Almasry <almasrymina@...gle.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Shuah Khan <shuah@...nel.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Jeroen de Borst <jeroendb@...gle.com>,
        Praveen Kaligineedi <pkaligineedi@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Kaiyuan Zhang <kaiyuanz@...gle.com>
Subject: Re: [RFC PATCH v3 05/12] netdev: netdevice devmem allocator

On 11/7/23 3:10 PM, Mina Almasry wrote:
> On Mon, Nov 6, 2023 at 3:44 PM David Ahern <dsahern@...nel.org> wrote:
>>
>> On 11/5/23 7:44 PM, Mina Almasry wrote:
>>> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
>>> index eeeda849115c..1c351c138a5b 100644
>>> --- a/include/linux/netdevice.h
>>> +++ b/include/linux/netdevice.h
>>> @@ -843,6 +843,9 @@ struct netdev_dmabuf_binding {
>>>  };
>>>
>>>  #ifdef CONFIG_DMA_SHARED_BUFFER
>>> +struct page_pool_iov *
>>> +netdev_alloc_devmem(struct netdev_dmabuf_binding *binding);
>>> +void netdev_free_devmem(struct page_pool_iov *ppiov);
>>
>> netdev_{alloc,free}_dmabuf?
>>
> 
> Can do.
> 
>> I say that because a dmabuf can be host memory, at least I am not aware
>> of a restriction that a dmabuf is device memory.
>>
> 
> In my limited experience dma-buf is generally device memory, and
> that's really its use case. CONFIG_UDMABUF is a driver that mocks
> dma-buf with a memfd which I think is used for testing. But I can do
> the rename, it's more clear anyway, I think.

config UDMABUF
        bool "userspace dmabuf misc driver"
        default n
        depends on DMA_SHARED_BUFFER
        depends on MEMFD_CREATE || COMPILE_TEST
        help
          A driver to let userspace turn memfd regions into dma-bufs.
          Qemu can use this to create host dmabufs for guest framebuffers.


Qemu is just a userspace process; it is no way a special one.

Treating host memory as a dmabuf should radically simplify the io_uring
extension of this set. That the io_uring set needs to dive into
page_pools is just wrong - complicating the design and code and pushing
io_uring into a realm it does not need to be involved in.

Most (all?) of this patch set can work with any memory; only device
memory is unreadable.