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

[Leonard Crestez]

On Mon, 2017-09-18 at 11:08 -0700, 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,
> >Â
> > 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.
>Â
> 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.

I am the one who reported this, on the #xenarm IRC channel.

Not implementing mmap in dma_map_ops means that dma_common_mmap is
called by dma_map_attrs as a fallback. The end result is not something
like -ENOSYS but what seem to be corrupt mappings.

However I agree that backporting might be excessive. I ran into this by
experimenting with using a GPU from dom0. It seems reasonable to get
kernel crashes if you try this kind of stuff.

This patch results in calling __swiotlb_mmap instead of
dma_common_mmap. I don't know the implementation details of the DMA api
but the interesting difference between these paths seems to be the way
pfn is fetched (from dma_addr instead of the kernel virt addr).

--
Regards,
Leonard