Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Fri, 19 Jul 2019 08:48:53 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Pavel Machek <pavel@ucw.cz>
Cc:     Christoph Hellwig <hch@infradead.org>, john.hubbard@gmail.com,
        SCheung@nvidia.com, akpm@linux-foundation.org,
        aneesh.kumar@linux.vnet.ibm.com, benh@kernel.crashing.org,
        bsingharora@gmail.com, dan.j.williams@intel.com,
        dnellans@nvidia.com, ebaskakov@nvidia.com, hannes@cmpxchg.org,
        jglisse@redhat.com, jhubbard@nvidia.com,
        kirill.shutemov@linux.intel.com, linux-kernel@vger.kernel.org,
        liubo95@huawei.com, mhairgrove@nvidia.com, mhocko@kernel.org,
        paulmck@linux.vnet.ibm.com, ross.zwisler@linux.intel.com,
        sgutti@nvidia.com, torvalds@linux-foundation.org,
        vdavydov.dev@gmail.com
Subject: Re: [PATCH] mm/Kconfig: additional help text for HMM_MIRROR option
Message-ID: <20190719114853.GB15816@ziepe.ca>
References: <20190717074124.GA21617@amd>
 <20190719013253.17642-1-jhubbard@nvidia.com>
 <20190719055748.GA29082@infradead.org>
 <20190719105239.GA10627@amd>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190719105239.GA10627@amd>
User-Agent: Mutt/1.9.4 (2018-02-28)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Jul 19, 2019 at 12:52:39PM +0200, Pavel Machek wrote:
> On Thu 2019-07-18 22:57:48, Christoph Hellwig wrote:
> > On Thu, Jul 18, 2019 at 06:32:53PM -0700, john.hubbard@gmail.com wrote:
> > > +	  HMM_MIRROR provides a way to mirror ranges of the CPU page tables
> > > +	  of a process into a device page table. Here, mirror means "keep
> > > +	  synchronized". Prerequisites: the device must provide the ability
> > > +	  to write-protect its page tables (at PAGE_SIZE granularity), and
> > > +	  must be able to recover from the resulting potential page faults.
> > > +
> > > +	  Select HMM_MIRROR if you have hardware that meets the above
> > > +	  description. An early, partial list of such hardware is:
> > > +	  an NVIDIA GPU >= Pascal, Mellanox IB >= mlx5, or an AMD GPU.
> > 
> > Nevermind that the Nvidia support is stagaging and looks rather broken,
> > there is no Mellanox user of this either at this point.
> > 
> > But either way this has no business in a common kconfig help.  Just
> > drop the fine grained details and leave it to the overview.
> 
> I disagree here. This explains what kind of hardware this is for (very
> new). Partial list does not hurt, and I know that I probably don't
> need to enable this.
> 
> How else am I supposed to know if my computer needs page tables
> synchronized?

It is like MMU_NOTIFIERS, if something needs it, then it will select
it.

Maybe it should just be a hidden kconfig anyhow as there is no reason
to turn it on without also turning on a using driver.

Jason