[AMD Official Use Only - AMD Internal Distribution Only]

HDP flush is done in ROCm using these 3 methods:

1. For AQL packets, this is done by setting the system-scope acquire and release fences in the packet header.
     For example, it is set here:
     https://github.com/ROCm/rocm-systems/blob/develop/projects/rocr-runtime/runtime/hsa-runtime/core/runtime/amd_blit_kernel.cpp#L878

     And when the headers are defined here:
     https://github.com/ROCm/rocm-systems/blob/develop/projects/clr/rocclr/device/rocm/rocvirtual.cpp#L85


2. Via a SDMA packet. This is done before doing a memory copy:
     The function is called here:
        https://github.com/ROCm/rocm-systems/blob/develop/projects/rocr-runtime/runtime/hsa-runtime/core/runtime/amd_blit_sdma.cpp#L484
     And the packet (POLL_REGMEM) is generated here:
        https://github.com/ROCm/rocm-systems/blob/develop/projects/rocr-runtime/runtime/hsa-runtime/core/runtime/amd_blit_sdma.cpp#L1154


3. By writing to a MMIO remapped address:
            The address is stored in rocclr here:
        https://github.com/ROCm/rocm-systems/blob/develop/projects/clr/rocclr/device/rocm/rocdevice.cpp#L607

            And the flush is triggered by writing a 1, e.g here:
        https://github.com/ROCm/rocm-systems/blob/develop/projects/clr/rocclr/device/rocm/rocvirtual.cpp#L3831

Hi David, Alex and Christian

Thank you for pointing me to the code where the HDP flush is handled.

From my understanding of the code, if MMIO remap is supported, the HDP flush is performed via MMIO remap. If MMIO remap is not supported, a workaround is used:
"write all arguments → sfence → write the last byte → mfence → read the last byte"

This sequence ensures that the HDP flush is completed before the GPU reads the data.

With this understanding, I think patch 5/8 (amdkfd/kfd_chardev: Add error message for non-4K page size failures) may not be needed, as it prints an error when MMIO remap is not supported, which is an expected case.

Should we drop patch 5/8, or do you think it would be better to change this to a pr_warn or pr_info message when MMIO remap is not supported?

Regards,
David

-----Original Message-----
From: Alex Deucher <alexdeucher@gmail.com>
Sent: Wednesday, December 17, 2025 9:23 AM
To: Donet Tom <donettom@linux.ibm.com>; Yat Sin, David
<David.YatSin@amd.com>
Cc: Koenig, Christian <Christian.Koenig@amd.com>; Ritesh Harjani (IBM)
<ritesh.list@gmail.com>; amd-gfx@lists.freedesktop.org; Kuehling, Felix
<Felix.Kuehling@amd.com>; Deucher, Alexander
<Alexander.Deucher@amd.com>; Russell, Kent <Kent.Russell@amd.com>;
Vaidyanathan Srinivasan <svaidy@linux.ibm.com>; Mukesh Kumar Chaurasiya
<mkchauras@linux.ibm.com>
Subject: Re: [RFC PATCH v1 0/8] amdgpu/amdkfd: Add support for non-4K page
size systems

On Wed, Dec 17, 2025 at 4:03 AM Donet Tom <donettom@linux.ibm.com> wrote:

On 12/16/25 7:32 PM, Alex Deucher wrote:

On Tue, Dec 16, 2025 at 8:55 AM Donet Tom <donettom@linux.ibm.com>

wrote:

On 12/15/25 7:39 PM, Alex Deucher wrote:

On Mon, Dec 15, 2025 at 4:47 AM Christian König
<christian.koenig@amd.com> wrote:

On 12/12/25 18:24, Alex Deucher wrote:

On Fri, Dec 12, 2025 at 8:19 AM Christian König
<christian.koenig@amd.com> wrote:

On 12/12/25 11:45, Ritesh Harjani (IBM) wrote:

Christian König <christian.koenig@amd.com> writes:

Setup details:
============
System details: Power10 LPAR using 64K pagesize.
AMD GPU:
    Name:                    gfx90a
    Marketing Name:          AMD Instinct MI210

Queries:
=======
1. We currently ran rocr-debug agent tests [1]  and rccl unit tests [2]

to test

     these changes. Is there anything else that you would suggest us

to run to

     shake out any other page size related issues w.r.t the kernel

driver?

The ROCm team needs to answer that.

Is there any separate mailing list or list of people whom we
can cc then?

With Felix on CC you already got the right person, but he's on vacation

and will not be back before the end of the year.

I can check on Monday if some people are still around which could

answer a couple of questions, but in general don't expect a quick response.

2. Patch 1/8: We have a querry regarding eop buffer size Is this eop

ring buffer

     size HW dependent? Should it be made PAGE_SIZE?

Yes and no.

If you could more elaborate on this please? I am assuming you
would anyway respond with more context / details on Patch-1
itself. If yes, that would be great!

Well, in general the EOP (End of Pipe) buffer contains in a ring buffer of

all the events and actions the CP should execute when shaders and cache flushes
finish.

The size depends on the HW generation and configuration of the GPU

etc..., but don't ask me for details how that is calculated.

The point is that the size is completely unrelated to the CPU, so using

PAGE_SIZE is clearly incorrect.

3. Patch 5/8: also have a query w.r.t the error paths when system

page size > 4K.

     Do we need to lift this restriction and add MMIO remap support

for systems with

     non-4K page sizes?

The problem is the HW can't do this.

We aren't that familiar with the HW / SW stack here. Wanted to
understand what functionality will be unsupported due to this HW

limitation then?

The problem is that the CPU must map some of the registers/resources

of the GPU into the address space of the application and you run into security
issues when you map more than 4k at a time.

Right.  There are some 4K pages with the MMIO register BAR which
are empty and registers can be remapped into them.  In this case
we remap the HDP flush registers into one of those register
pages.  This allows applications to flush the HDP write FIFO
from either the CPU or another device.  This is needed to flush
data written by the CPU or another device to the VRAM BAR out to
VRAM (i.e., so the GPU can see it).  This is flushed internally
as part of the shader dispatch packets,

As far as I know this is only done for graphics shader submissions to the

classic CS interface, but not for compute dispatches through ROCm queues.

There is an explicit PM4 packet to flush the HDP cache for userqs
and for AQL the flush is handled via one of the flags in the
dispatch packet.  The MMIO remap is needed for more fine grained
use cases where you might have the CPU or another device operating
in a gang like scenario with the GPU.

Thank you, Alex.

We were encountering an issue while running the RCCL unit tests.
With 2 GPUs, all tests passed successfully; however, when running
with more than 2 GPUs, the tests began to fail at random points
with the following
errors:

[  598.576821] amdgpu 0048:0f:00.0: amdgpu: Queue preemption failed
for queue with doorbell_id: 80030008 [  606.696820] amdgpu
0048:0f:00.0: amdgpu: Failed to evict process queues [  606.696826]
amdgpu 0048:0f:00.0: amdgpu: GPU reset begin!. Source: 4 [
610.696852] amdgpu 0048:0f:00.0: amdgpu: Queue preemption failed
for queue with doorbell_id: 80030008 [  610.696869] amdgpu
0048:0f:00.0: amdgpu: Failed to evict process queues [  610.696942]
amdgpu 0048:0f:00.0: amdgpu: Failed to restore process queues


After applying patches 7/8 and 8/8, we are no longer seeing this issue.

One question I have is: we only started observing this problem when
the number of GPUs increased. Could this be related to MMIO
remapping not being available?

It could be.  E.g., if the CPU or a GPU writes data to VRAM on
another GPU, you will need to flush the HDP to make sure that data
hits VRAM before the GPU attached to the VRAM can see it.

Thanks Alex

I am now suspecting that the queue preemption issue may be related to
the unavailability of MMIO remapping. I am not very familiar with this area.

Could you please point me to the relevant code path where the PM4
packet is issued to flush the HDP cache?

+ David who is more familiar with the ROCm runtime.

PM4 has a packet called HDP_FLUSH which flushes the HDP.  For AQL, it's
handled by one of the flags I think.  Most things in ROCm use AQL.

@David Yat Sin Can you point to how HDP flushes are handled in the ROCm
runtime?

Alex

I am consistently able to reproduce this issue on my system when using
more than three GPUs if patches 7/8 and 8/8 are not applied. In your
opinion, is there anything that can be done to speed up the HDP flush
or to avoid this situation altogether?

Alex

Alex

That's the reason why ROCm needs the remapped MMIO register BAR.

but there are certain cases where an application may want more
control.  This is probably not a showstopper for most ROCm apps.

Well the problem is that you absolutely need the HDP flush/invalidation for

100% correctness. It does work most of the time without it, but you then risk data
corruption.

Apart from making the flush/invalidate an IOCTL I think we could also just

use a global dummy page in VRAM.

If you make two 32bit writes which are apart from each other and then a

read back a 32bit value from VRAM that should invalidate the HDP as well. It's less
efficient than the MMIO BAR remap but still much better than going though an
IOCTL.

The only tricky part is that you need to get the HW barriers with the doorbell

write right.....

That said, the region is only 4K so if you allow applications to
map a larger region they would get access to GPU register pages
which they shouldn't have access to.

But don't we also have problems with the doorbell? E.g. the global

aggregated one needs to be 4k as well, or is it ok to over allocate there?

Thinking more about it there is also a major problem with page tables.

Those are 4k by default on modern systems as well and while over allocating them
to 64k is possible that not only wastes some VRAM but can also result in OOM
situations because we can't allocate the necessary page tables to switch from 2MiB
to 4k pages in some cases.

Christian.

Alex

[1] ROCr debug agent tests:
https://github.com/ROCm/rocr_debug_agent
[2] RCCL tests:
https://github.com/ROCm/rccl/tree/develop/test


Please note that the changes in this series are on a best
effort basis from our end. Therefore, requesting the amd-gfx
community (who have deeper knowledge of the HW & SW stack)
to kindly help with the review and provide feedback /
comments on these patches. The idea here is, to also have non-4K

pagesize (e.g. 64K) well supported with amd gpu kernel driver.

Well this is generally nice to have, but there are unfortunately some

HW limitations which makes ROCm pretty much unusable on non 4k page size
systems.

That's a bummer :(
- Do we have some HW documentation around what are these

limitations around non-4K pagesize? Any links to such please?

You already mentioned MMIO remap which obviously has that problem,

but if I'm not completely mistaken the PCIe doorbell BAR and some global seq
counter resources will also cause problems here.

This can all be worked around by delegating those MMIO accesses into

the kernel, but that means tons of extra IOCTL overhead.

Especially the cache flushes which are necessary to avoid corruption

are really bad for performance in such an approach.

- Are there any latest AMD GPU versions which maybe lifts such

restrictions?

Not that I know off any.

What we can do is to support graphics and MM, but that should

already work out of the box.

- Maybe we should also document, what will work and what won't work

due to these HW limitations.

Well pretty much everything, I need to double check how ROCm does

HDP flushing/invalidating when the MMIO remap isn't available.

Could be that there is already a fallback path and that's the reason why

this approach actually works at all.

What we can do is to support graphics and MM, but that should
already work out of the box.>

So these patches helped us resolve most of the issues like
SDMA hangs and GPU kernel page faults which we saw with rocr
and rccl tests with 64K pagesize. Meaning, we didn't see this
working out of box perhaps due to 64K pagesize.

Yeah, but this is all for ROCm and not the graphics side.

To be honest I'm not sure how ROCm even works when you have 64k

pages at the moment. I would expect much more issue lurking in the kernel driver.

AFAIU, some of these patches may require re-work based on
reviews, but at least with these changes, we were able to see all the

tests passing.

I need to talk with Alex and the ROCm team about it if workarounds

can be implemented for those issues.

Thanks a lot! That would be super helpful!

Regards,
Christian.

Thanks again for the quick response on the patch series.

You are welcome, but since it's so near to the end of the year not all

people are available any more.

Regards,
Christian.

-ritesh