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

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Date:   Mon, 18 Sep 2017 11:08:03 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Greg KH <gregkh@...uxfoundation.org>
cc:     Stefano Stabellini <sstabellini@...nel.org>,
        stable@...r.kernel.org, linux-kernel@...r.kernel.org,
        julien.grall@....com, jgross@...e.com, boris.ostrovsky@...cle.com
Subject: Re: [BACKPORT] swiotlb-xen: implement xen_swiotlb_dma_mmap
 callback

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.