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Date:   Wed, 29 Apr 2020 19:41:27 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Felix Kuehling <felix.kuehling@amd.com>
Cc:     linux-mm@kvack.org, Ralph Campbell <rcampbell@nvidia.com>,
        "Yang, Philip" <Philip.Yang@amd.com>,
        Alex Deucher <alexander.deucher@amd.com>,
        amd-gfx@lists.freedesktop.org, Ben Skeggs <bskeggs@redhat.com>,
        Christian =?utf-8?B?S8O2bmln?= <christian.koenig@amd.com>,
        "David (ChunMing) Zhou" <David1.Zhou@amd.com>,
        dri-devel@lists.freedesktop.org, Christoph Hellwig <hch@lst.de>,
        intel-gfx@lists.freedesktop.org,
        =?utf-8?B?SsOpcsO0bWU=?= Glisse <jglisse@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>,
        linux-kernel@vger.kernel.org,
        Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>,
        nouveau@lists.freedesktop.org
Subject: Re: [PATCH hmm 5/5] mm/hmm: remove the customizable pfn format from
 hmm_range_fault
Message-ID: <20200429224127.GB3824@ziepe.ca>
References: <5-v1-4eb72686de3c+5062-hmm_no_flags_jgg@mellanox.com>
 <ec276ac2-a346-5728-4ac6-9c9bd9ffcd41@amd.com>
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On Wed, Apr 22, 2020 at 01:52:32PM -0400, Felix Kuehling wrote:
> [+Philip Yang]
> 
> Am 2020-04-21 um 8:21 p.m. schrieb Jason Gunthorpe:
> > From: Jason Gunthorpe <jgg@mellanox.com>
> >
> > Presumably the intent here was that hmm_range_fault() could put the data
> > into some HW specific format and thus avoid some work. However, nothing
> > actually does that, and it isn't clear how anything actually could do that
> > as hmm_range_fault() provides CPU addresses which must be DMA mapped.
> >
> > Perhaps there is some special HW that does not need DMA mapping, but we
> > don't have any examples of this, and the theoretical performance win of
> > avoiding an extra scan over the pfns array doesn't seem worth the
> > complexity. Plus pfns needs to be scanned anyhow to sort out any
> > DEVICE_PRIVATE pages.
> >
> > This version replaces the uint64_t with an usigned long containing a pfn
> > and fix flags. On input flags is filled with the HMM_PFN_REQ_* values, on
> > successful output it is filled with HMM_PFN_* values, describing the state
> > of the pages.
> >
> > amdgpu is simple to convert, it doesn't use snapshot and doesn't use
> > per-page flags.
> >
> > nouveau uses only 16 hmm_pte entries at most (ie fits in a few cache
> > lines), and it sweeps over its pfns array a couple of times anyhow.
> >
> > Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
> > Signed-off-by: Christoph Hellwig <hch@lst.de>
> 
> Hi Jason,
> 
> I pointed out a typo in the documentation inline. Other than that, the
> series is
> 
> Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
> 
> I'll try to build it and run some basic tests later.

Got it, thanks! Let me know if there are problems

Jason