Re: [HMM-CDM 0/5] Cache coherent device memory (CDM) with HMM

[Balbir Singh]

On Thu, Jun 15, 2017 at 7:58 AM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> On 06/14/2017 02:38 PM, Jerome Glisse wrote:
>> On Wed, Jun 14, 2017 at 02:20:23PM -0700, Dave Hansen wrote:
>>> On 06/14/2017 01:11 PM, JÃrÃme Glisse wrote:
>>>> Cache coherent device memory apply to architecture with system bus
>>>> like CAPI or CCIX. Device connected to such system bus can expose
>>>> their memory to the system and allow cache coherent access to it
>>>> from the CPU.
>>> How does this interact with device memory that's enumerated in the new
>>> ACPI 6.2 HMAT?  That stuff is also in the normal e820 and, by default,
>>> treated as normal system RAM.  Would this mechanism be used for those
>>> devices as well?
>>>
>>> http://www.uefi.org/sites/default/files/resources/ACPI_6_2.pdf
>> It doesn't interact with that. HMM-CDM is a set of helper that don't
>> do anything unless instructed so. So for device memory to be presented
>> as HMM-CDM you need to hotplug it as ZONE_DEVICE(DEVICE_PUBLIC) which
>> can be done with the helper introduced in patch 2 of this patchset.
>

[Removing my cc'd email id and responding from a different address]

> I guess I'm asking whether we *should* instruct HMM-CDM to manage all
> coherent device memory.  If not, where do we draw the line for what we
> use HMM-CDM, and for what we use the core MM?
>

If you believe the memory is managed by the device (and owned by a device
driver) I'd suggest using HMM-CDM. The idea behind HMM-CDM was that
it enables transparent migration of pages and its preferred when
locality of computation
and locality of memory access is the preferred model.

The other model was N_COHERENT_MEMORY that used the core MM, but
there were objections to exposing device memory using that technology.

Balbir Singh.