netdev - Re: [PATCH net-next v2 4/5] page_pool: disable sync for cpu for dmabuf memory provider

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Message-ID: <39597ca8-11e4-4c1d-9970-1c73f1f5c62d@gmail.com>
Date: Fri, 8 Nov 2024 16:39:34 +0000
From: Pavel Begunkov <asml.silence@...il.com>
To: Stanislav Fomichev <stfomichev@...il.com>, Jason Gunthorpe <jgg@...pe.ca>
Cc: Mina Almasry <almasrymina@...gle.com>, netdev@...r.kernel.org,
 Jakub Kicinski <kuba@...nel.org>, Willem de Bruijn <willemb@...gle.com>,
 Kaiyuan Zhang <kaiyuanz@...gle.com>, Samiullah Khawaja
 <skhawaja@...gle.com>, linux-kernel@...r.kernel.org,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>,
 Jesper Dangaard Brouer <hawk@...nel.org>,
 Ilias Apalodimas <ilias.apalodimas@...aro.org>
Subject: Re: [PATCH net-next v2 4/5] page_pool: disable sync for cpu for
 dmabuf memory provider

On 11/8/24 15:58, Stanislav Fomichev wrote:
> On 11/08, Jason Gunthorpe wrote:
>> On Thu, Nov 07, 2024 at 09:23:08PM +0000, Mina Almasry wrote:
>>> dmabuf dma-addresses should not be dma_sync'd for CPU/device. Typically
>>> its the driver responsibility to dma_sync for CPU, but the driver should
>>> not dma_sync for CPU if the netmem is actually coming from a dmabuf
>>> memory provider.
>>
>> This is not completely true, it is not *all* dmabuf, just the parts of
>> the dmabuf that are actually MMIO.
>>
>> If you do this you may want to block accepting dmabufs that have CPU
>> pages inside them.
> 
> We still want udmabufs to work, so probably need some new helper to test
> whether a particular netmem is backed by the cpu memory?

Agree. I guess it's fair to assume that page pool is backed either
by one or another, so could be a page pool flag that devmem.c can set
on init.

-- 
Pavel Begunkov