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

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Message-ID: <alpine.DEB.2.10.1709191453500.2968@sstabellini-ThinkPad-X260>
Date:   Tue, 19 Sep 2017 14:55:22 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
cc:     Stefano Stabellini <sstabellini@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>, 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 Mon, 18 Sep 2017, Boris Ostrovsky wrote:
> 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).

arch/arm/mm/dma-mapping.c:arm_dma_mmap
arch/arm/mm/dma-mapping.c:arm_coherent_dma_mmap
arch/arm64/mm/dma-mapping.c:__swiotlb_mmap