linux-kernel - Re: dma_mmap

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Date:   Tue, 10 Sep 2019 10:37:15 +0200
From:   Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        "pv-drivers@...are.com" <pv-drivers@...are.com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Christian König <christian.koenig@....com>
Subject: Re: dma_mmap_fault discussion

On 9/6/19 9:20 AM, Christoph Hellwig wrote:
> On Fri, Sep 06, 2019 at 09:10:08AM +0200, Thomas Hellström (VMware) wrote:
>> It's definitely possible. I was just wondering whether it was necessary, but
>> it seems like it.
> Yepp.
>
> I've pushed a new version out (even hotter off the press) that doesn't
> require the region for dma_mmap_prepare, and also uses PAGE_SIZE units
> for the offset / length in dma_mmap_fault, which simplifies a few
> things.

Hi, Christoph,

I've been looking into this a bit, and while it's possible to make it 
work for fault, (and for single page kmaps of course, since we have the 
kernel virtual address already), I run into problems with vmaps, since 
we basically need to vmap a set of consecutive coherent allocations 
obtained from the coherent pool.

Also, at mmap time, we need in theory to call dma_mmap_prepare() and 
save the page_prot for all attrs we might be using at fault time...

I did some thinking into whether we could perhaps cover all or at least 
the vast majority of architectures and awkward devices more generally 
with a  page prot and a set of pfns. So I looked at how 
remap_pfn_range() and io_remap_pfn_range() was implemented across 
architectures, and it turns out that all archs implementing a special 
io_remap_pfn_range() (sparc and mips) end up calling remap_pfn_range(), 
and that should mean that any arch that's currently capable of doing 
fault() should in principle be capable of using vmf_insert_pfn_prot() 
with a suitable pfn that in most (if not all) cases should be obtainable 
from the kernel virtual address.

So do you think a way forward could perhaps be to have a 
dma_common_get_pfn_sgtable() and add a generic vmap_pfn_prot()?

Thanks,

Thomas