linux-kernel - Re: [BACKPORT] swiotlb-xen: implement xen_swiotlb_dma

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Message-ID: <20abb053-64fc-c7a3-066f-6d1491f13108@oracle.com>
Date:   Mon, 18 Sep 2017 14:55:54 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Stefano Stabellini <sstabellini@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>
Cc:     stable@...r.kernel.org, linux-kernel@...r.kernel.org,
        julien.grall@....com, jgross@...e.com
Subject: Re: [BACKPORT] swiotlb-xen: implement xen_swiotlb_dma_mmap callback

On 09/18/2017 02:08 PM, Stefano Stabellini wrote:
> On Fri, 15 Sep 2017, Greg KH wrote:
>> On Thu, Sep 14, 2017 at 04:23:05PM -0700, Stefano Stabellini wrote:
>>> Hi all,
>>>
>>> We are getting reports from Xen on ARM users about DMA issues. The
>>> problem is that the commit below
>>> (7e91c7df29b5e196de3dc6f086c8937973bd0b88) is necessary to support mmap
>>> on Xen on ARM. It is self-contained and doesn't affect anything outside
>>> of Xen on ARM, so I think is a good candidate for backporting. It went
>>> upstream in 4.11.
>> But it's a new feature, right?  How does that fit the stable kernel
>> rules?
> It implements a previously unimplemented function (mmap), although it
> calls the generic functions to do it. Yes, I agree with you that it
> can be classified as a new feature. If that is against the stable kernel
> rules, then please discard this request.
>
> FYI the reason why it didn't raise a flag in my mind is that users
> reported something like "unhandled alignment fault (11) at
> 0xffffa6048080, esr 0x92000061", which really looks more like a bug.
>
>
>>> Could you please backport the following commit:
>>>
>>>   commit 7e91c7df29b5e196de3dc6f086c8937973bd0b88
>>>   Author: Stefano Stabellini <stefano.stabellini@...citrix.com>
>>>   Date:   Tue Feb 7 19:58:02 2017 +0200
>>>   
>>>       swiotlb-xen: implement xen_swiotlb_dma_mmap callback
>>>       
>>>       This function creates userspace mapping for the DMA-coherent memory.
>>>     
>>> to the stable trees up until 3.14?
>>>
>>>
>>> Because of 00085f1efa387a8ce100e3734920f7639c80caa3 "dma-mapping: use
>>> unsigned long for dma_attrs", the appended patch (to be applied on top)
>>> is required for trees older than 4.8. 
>> What does the kvm maintainers think about this?
> That would be the Xen maintainers right? In that case, Boris, Juergen,
> please let us know what you think.


This is a nop for x86 so it's safe from that perspective. I can't find
mmap op for ARM though (xen_get_dma_ops(dev)->mmap).

-boris