Re: Enabling peer to peer device transactions for PCIe devices

On Tue, Nov 22, 2016 at 1:03 PM, Daniel Vetter [off-list ref] wrote:

On Tue, Nov 22, 2016 at 9:35 PM, Serguei Sagalovitch
[off-list ref] wrote:

quoted

On 2016-11-22 03:10 PM, Daniel Vetter wrote:

quoted

On Tue, Nov 22, 2016 at 9:01 PM, Dan Williams [off-list ref]
wrote:

quoted

On Tue, Nov 22, 2016 at 10:59 AM, Serguei Sagalovitch
[off-list ref] wrote:

quoted

I personally like "device-DAX" idea but my concerns are:

-  How well it will co-exists with the  DRM infrastructure /
implementations
    in part dealing with CPU pointers?

Inside the kernel a device-DAX range is "just memory" in the sense
that you can perform pfn_to_page() on it and issue I/O, but the vma is
not migratable. To be honest I do not know how well that co-exists
with drm infrastructure.

quoted

-  How well we will be able to handle case when we need to
"move"/"evict"
    memory/data to the new location so CPU pointer should point to the
new
physical location/address
     (and may be not in PCI device memory at all)?

So, device-DAX deliberately avoids support for in-kernel migration or
overcommit. Those cases are left to the core mm or drm. The device-dax
interface is for cases where all that is needed is a direct-mapping to
a statically-allocated physical-address range be it persistent memory
or some other special reserved memory range.

For some of the fancy use-cases (e.g. to be comparable to what HMM can
pull off) I think we want all the magic in core mm, i.e. migration and
overcommit. At least that seems to be the very strong drive in all
general-purpose gpu abstractions and implementations, where memory is
allocated with malloc, and then mapped/moved into vram/gpu address
space through some magic,

It is possible that there is other way around: memory is requested to be
allocated and should be kept in vram for  performance reason but due
to possible overcommit case we need at least temporally to "move" such
allocation to system memory.

With migration I meant migrating both ways of course. And with stuff
like numactl we can also influence where exactly the malloc'ed memory
is allocated originally, at least if we'd expose the vram range as a
very special numa node that happens to be far away and not hold any
cpu cores.

I don't think we should be using numa distance to reverse engineer a
certain allocation behavior.  The latency data should be truthful, but
you're right we'll need a mechanism to keep general purpose
allocations out of that range by default. Btw, strict isolation is
another design point of device-dax, but I think in this case we're
describing something between the two extremes of full isolation and
full compatibility with existing numactl apis.