RE: [PATCH 00/83] AMD HSA kernel driver

From: Bridgman, John
Date: Tue Jul 15 2014 - 13:53:50 EST

>-----Original Message-----
>From: Jerome Glisse [mailto:j.glisse@xxxxxxxxx]
>Sent: Tuesday, July 15, 2014 1:37 PM
>To: Bridgman, John
>Cc: Dave Airlie; Christian König; Lewycky, Andrew; linux-
>kernel@xxxxxxxxxxxxxxx; dri-devel@xxxxxxxxxxxxxxxxxxxxx; Deucher,
>Alexander; akpm@xxxxxxxxxxxxxxxxxxxx
>Subject: Re: [PATCH 00/83] AMD HSA kernel driver
>On Tue, Jul 15, 2014 at 05:06:56PM +0000, Bridgman, John wrote:
>> >From: Dave Airlie [mailto:airlied@xxxxxxxxx]
>> >Sent: Tuesday, July 15, 2014 12:35 AM
>> >To: Christian König
>> >Cc: Jerome Glisse; Bridgman, John; Lewycky, Andrew; linux-
>> >kernel@xxxxxxxxxxxxxxx; dri-devel@xxxxxxxxxxxxxxxxxxxxx; Deucher,
>> >Alexander; akpm@xxxxxxxxxxxxxxxxxxxx
>> >Subject: Re: [PATCH 00/83] AMD HSA kernel driver
>> >
>> >On 14 July 2014 18:37, Christian König <deathsimple@xxxxxxxxxxx> wrote:
>> >>> I vote for HSA module that expose ioctl and is an intermediary
>> >>> with the kernel driver that handle the hardware. This gives a
>> >>> single point for HSA hardware and yes this enforce things for any
>> >>> hardware
>> >manufacturer.
>> >>> I am more than happy to tell them that this is it and nothing else
>> >>> if they want to get upstream.
>> >>
>> >> I think we should still discuss this single point of entry a bit more.
>> >>
>> >> Just to make it clear the plan is to expose all physical HSA
>> >> capable devices through a single /dev/hsa device node to userspace.
>> >
>> >This is why we don't design kernel interfaces in secret foundations,
>> >and expect anyone to like them.
>> Understood and agree. In this case though this isn't a cross-vendor
>> interface designed by a secret committee, it's supposed to be more of
>> an inoffensive little single-vendor interface designed *for* a secret
>> committee. I'm hoping that's better ;)
>> >
>> >So before we go any further, how is this stuff planned to work for
>> >multiple GPUs/accelerators?
>> Three classes of "multiple" :
>> 1. Single CPU with IOMMUv2 and multiple GPUs:
>> - all devices accessible via /dev/kfd
>> - topology information identifies CPU + GPUs, each has "node ID" at
>> top of userspace API, "global ID" at user/kernel interface (don't
>> think we've implemented CPU part yet though)
>> - userspace builds snapshot from sysfs info & exposes to HSAIL
>> runtime, which in turn exposes the "standard" API
>This is why i do not like the sysfs approach, it would be lot nicer to have
>device file per provider and thus hsail can listen on device file event and
>discover if hardware is vanishing or appearing. Periodicaly going over sysfs
>files is not the right way to do that.

Agree that wouldn't be good. There's an event mechanism still to come - mostly for communicating fences and shader interrupts back to userspace, but also used for "device change" notifications, so no polling of sysfs.

>> - kfd sets up ATC aperture so GPUs can access system RAM via IOMMUv2
>> (fast for APU, relatively less so for dGPU over PCIE)
>> - to-be-added memory operations allow allocation & residency control
>> (within existing gfx driver limits) of buffers in VRAM & carved-out
>> system RAM
>> - queue operations specify a node ID to userspace library, which
>> translates to "global ID" before calling kfd
>> 2. Multiple CPUs connected via fabric (eg HyperTransport) each with 0 or
>more GPUs:
>> - topology information exposes CPUs & GPUs, along with affinity info
>> showing what is connected to what
>> - everything else works as in (1) above
>This is suppose to be part of HSA ? This is lot broader than i thought.

Yes although it can be skipped on most systems. We figured that topology needed to cover everything that would be handled by a single OS image, so in a NUMA system it would need to cover all the CPUs. I think that is still the right scope, do you agree ?

>> 3. Multiple CPUs not connected via fabric (eg a blade server) each
>> with 0 or more GPUs
>> - no attempt to cover this with HSA topology, each CPU and associated
>> GPUs is accessed independently via separate /dev/kfd instances
>> >
>> >Do we have a userspace to exercise this interface so we can see how
>> >such a thing would look?
>> Yes -- initial IP review done, legal stuff done, sanitizing WIP,
>> hoping for final approval this week
>> There's a separate test harness to exercise the userspace lib calls,
>> haven't started IP review or sanitizing for that but legal stuff is
>> done
>> >
>> >Dave.
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