netdev - Re: igb driver can cause cache invalidation of non-owned memory?

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Date:   Mon, 10 Oct 2016 12:51:28 +0300
From:   Nikita Yushchenko <nikita.yoush@...entembedded.com>
To:     David Miller <davem@...emloft.net>
Cc:     jeffrey.t.kirsher@...el.com, intel-wired-lan@...ts.osuosl.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        cphealy@...il.com
Subject: Re: igb driver can cause cache invalidation of non-owned memory?

>> With this scheme, page used for Rx is completely dma_map()ed at
>> allocation time, split into two buffers, and individual buffer is
>> sync_to_cpu()ed AND PASSED TO NETWORK STACK via skb_add_rx_frag() -
>> while driver driver still uses other buffer. Later, when driver decides
>> to no longer use this page, it will dma_unmap() it completely - which on
>> archs with non-coherent caches means cache invalidation. This cache
>> invalidation will include area that is already passed elsewhere.
> 
> This should happen only if the device wrote into that piece of the
> memory which it absolutely should not.

Hmm...  I'm not about device writing to memory.


Sequence in igb driver is:

dma_map(full_page)
  <device writes here>
sync_to_cpu(half_page);
skb_add_rx_frag(skb, half_page);
napi_gro_receive(skb);
  ...
dma_unmap(full_page)

What I'm concerned about is - same area is first passed up to network
stack, and _later_ dma_unmap()ed.  Is this indeed safe?


Nikita