Re: [BACKPORT] swiotlb-xen: implement xen_swiotlb_dma_mmap callback

From: Boris Ostrovsky
Date: Mon Sep 18 2017 - 14:56:24 EST

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@xxxxxxxxxxxxx>
>>> 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).