Re: [PATCH v7 07/12] dma-mapping: introduce dma_has_iommu()

[Dan Williams <dan.j.williams@intel.com>]

On Fri, Oct 6, 2017 at 4:10 PM, David Woodhouse <dwmw2@infradead.org> wrote:
> On Fri, 2017-10-06 at 15:52 -0700, Dan Williams wrote:
>> On Fri, Oct 6, 2017 at 3:45 PM, David Woodhouse <dwmw2@infradead.org> wrote:
>> >
>> > On Fri, 2017-10-06 at 15:35 -0700, Dan Williams wrote:
>> > >
>> > > Add a helper to determine if the dma mappings set up for a given device
>> > > are backed by an iommu. In particular, this lets code paths know that a
>> > > dma_unmap operation will revoke access to memory if the device can not
>> > > otherwise be quiesced. The need for this knowledge is driven by a need
>> > > to make RDMA transfers to DAX mappings safe. If the DAX file's block map
>> > > changes we need to be to reliably stop accesses to blocks that have been
>> > > freed or re-assigned to a new file.
>> > "a dma_unmap operation revoke access to memory"... but it's OK that the
>> > next *map* will give the same DMA address to someone else, right?
>>
>> I'm assuming the next map will be to other physical addresses and a
>> different requester device since the memory is still registered
>> exclusively.
>
> I meant the next map for this device/group.
>
> It may well use the same virtual DMA address as the one you just
> unmapped, yet actually map to a different physical address. So if the
> DMA still occurs to the "old" address, that isn't revoked at all — it's
> just going to the wrong physical location.
>
> And if you are sure that the DMA will never happen, why do you need to
> revoke the mapping in the first place?

Right, crossed mails. The semantic I want is that the IOVA is
invalidated / starts throwing errors to the device because the address
it thought it was talking to has been remapped in the file. Once
userspace wakes up and responds to this invalidation event it can do
the actual unmap to make the IOVA reusable again.
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