Re: [PATCH 0/6] drm/amdgpu: Improve retry fault handling

["Timur Kristóf" <timur.kristof@gmail.com>]

Hi Amir,

I pushed a work in progress branch here:
https://gitlab.freedesktop.org/Venemo/linux/-/commits/ven_retry_faults_navi4

This contains the two series that I sent to the mailing list yesterday:
"Improve retry fault handling"
"Improve soft IH ring"

If we have to rely on the soft IH ring on Navi4, then we absolutely need the 
patches from "Improve soft IH ring", as it fixes a few bugs and switches the 
soft IH ring to use the system unbound workqueue. Otherwise it can happen that 
the soft IH ring fills up even before your CPU can process anything in it. 
(This is especially bad on HW that doesn't have the filter CAM enabled,
eg. Strix Halo.)

On top of that,

There is a patch to solve the race condition and make sure that the retry CAM 
is ACKed after the VM update is complete. On Navi 31, this works and 
successfully makes sure that the race is eliminated and each VM fault is only 
processed once. I'd like to submit these if you guys agree on the approach.

And finally, there is a very messy patch that contains what I tried to do to 
get it all working on Navi 48. This has a lot of comments with my notes during 
development, and some attempts I tried to fix the issue (without success).

Basically, what I did was:

- Enable the retry CAM the same way as I did on IH 6.0 exactly like you said
- Added the cam_index to gmc_v12_0_process_interrupt
- Added a few bits to gfxhub_v12_0 and gfx_v12_0 to actually enable retry 
faults (without that, the faults don't have the retry bit in src_data[1])

Now, what I observe on Navi 48 is:

1. Retry fault interrupt is triggered
2. Dispatched on the soft IH ring
3. The interrupt handler is called again on the soft IRQ ring
4. The fault is mitigated by amdgpu_vm_handle_fault()
5. Finally, the retry CAM is ACKed
6. As soon as the CAM is ACKed, I get another interrupt for the same address, 
and it goes on infinitely or until GPU reset is triggered.

Attempts at fixing what I saw on Navi 48:

- Tried to add a TLB flush to various places. Now that the race is mitigated, 
the TLB flush doesn't make things worse anymore, but it's also not helping.
- Tried to set the FORCE_MISS bits on various cache related registers, hoping 
that it would work around the possibility of the PTE/PDE not being updated in 
the cache. This didn't help either.
- Tried to use amdgpu.vm_update_mode=3 to use the CPU to update the page 
tables, but that didn't help on Navi 48
- Tried to halt the CP using CP_ME_CNTL to see if it would stop sending 
interrupts, but it didn't.
- Using umr I tried to disable the L2 cache and reset it with the GRBM, but 
that just broke the whole system.

Other notes on the retry fault topic in general:

- On GPUs that don't have the retry CAM (eg. Navi 1-2 and APUs) we'll need to 
consider disabling the VM fault interrupt while processing the retry, because 
the soft IH ring can fill up really quickly and the CPU may struggle to keep up 
with it.
- In general (for all GPUs), I think it would be better to update a larger VA 
range of subsequent pages rather than just a single page, in order to more 
efficiently mitigate VM faults to neighbouring pages.
- Currently all of the invalid accesses are redirected to the same dummy page, 
which is a security hole because it means that a process that had an out of 
bounds write can leak data to a process that does an out of bounds read. I 
already talked to Christian about this, he has a few ideas how to fix it.

Thanks & best regards,
Timur


On Thursday, May 14, 2026 5:04:00 PM Central European Summer Time Shetaia, 
Amir wrote:
> AMD General
> 
> Hi Timur,
> 
> Thanks for clarifying. yes, please push the Navi 48 WIP branch when you have
> a chance, useful to compare side-by-side.
 
> Patch 5 / WDOORBELL on ih_v7_0: I have been able to confirm that doorbell
> ACK does not free the CAM slot on the NV4 family
 (gfx1200/1201). I
> verified empirically, slots walk monotonically until the CAM fills, then HW
> silently drops retry events. MMIO write to regIH_RETRY_CAM_ACK does free
> it. Almost certainly required for your Navi 48 attempt. 
> Why PTE update doesn't reach UTC L0: Most likely a race between the SDMA
> PTE-update job completing and the CAM ACK firing the
 IRQ for re-handling.
> Until SDMA finishes the write + invalidation propagates, UTC L0 keeps
> getting the stale "invalid" reply which is exactly your symptom. 
> To inspect with umr: clone https://gitlab.freedesktop.org/tomstdenis/umr and
> use scripts/diag/dump_all_cpc_info.sh (needs
 halt_if_hws_hang=1
> gpu_recovery=0). For UTC specifically: umr -i <gpu> -O bits -r
> '*.*.regGCVM_L2_STATUS' (look at L2_BUSY and CONTEXT_DOMAIN_BUSY per-VMID
> bits) and regGCVM_L2_PROTECTION_FAULT_STATUS (empty + busy = translation
> timeout, not denial). Page-table walk to confirm PTE landed: sudo umr -i
> <gpu> -vm <vmid>@<va> 1. 
> Your ACK-after-SDMA-fence idea: good catch. We hadn't dug into that race
> because the BO-clear deadlock dominates on NV4, but
 the ordering bug is
> almost certainly there too. Fence callback is the right shape, cleaner than
> blocking the worker. Would be a generic fix benefiting both Navi 3 and 4.
> Happy to review if you take a swing. 
> IH1 routing: fair point. I just inherited the existing IH0 path and haven't
> tried IH1. On the list once CAM ACK and fence ordering are stable.
 
> AMIR SHETAIA
> Senior Software Development Engineer  |  AMD
> Software Platform Architecture Team
> ----------------------------------------------------------------------------
> ------------------------------------------------------
 1 Commerce Valley
> Drive, Markham, ON L3T 7X6
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> 
> 
> 
> 
> -----Original Message-----
> From: Timur Kristóf <timur.kristof@gmail.com>
> Sent: Wednesday, May 13, 2026 6:12 PM
> To: Alex Deucher <alexdeucher@gmail.com>; Shetaia, Amir
> <Amir.Shetaia@amd.com>
 Cc: amd-gfx@lists.freedesktop.org; Deucher,
> Alexander <Alexander.Deucher@amd.com>; Koenig, Christian
> <Christian.Koenig@amd.com>; Marek Olšák <maraeo@gmail.com>; Natalie Vock
> <natalie.vock@gmx.de>; Melissa Wen <mwen@igalia.com> Subject: Re: [PATCH
> 0/6] drm/amdgpu: Improve retry fault handling 
> Hi Amir,
> 
> 
> > Timur, you are right, I see your patch 6 already does the MMIO ACK for
> > gmc_v11_0/ih_v6_0. I missed that. The gap is only in patch 5's ih_v7_0
> > implementation, which still does WDOORBELL. that's where I'd suggest
> > swapping in MMIO for NV4.
> 
> 
> First, let me clear up a slight misunderstanding here. The patch series that
> I sent here only contains what I managed to get working on Navi 31, it does
> not contain any code from my attempt at Navi 48.
 
> The patch 5 is just a slight refactor of the pre-existing code and
> (intentionally) doesn't contain any functional changes. It seems that IH 7.1
> relies on the doorbell, so I didn't want to remove it, albeit I have no
> means to verify if that actually works or not.
 
> If you like, I can push a few WIP patches to a different branch tomorrow to
> show you exactly what I did on Navi 48. However, I wouldn't want to submit
> that to the mailing list without first making sure that it works well.
 
> 
> > 1. "Fault never resolves on NV48" different shape from our
> > broken-CAM-ACK symptom.
> 
> 
> 
> > You're right, those are different. Our cam-walk-monotonically symptom
> > only shows up when CAM is enabled but the ACK is broken.
> > 
>  > On your NV48 setup CAM
> > 
> > probably isn't enabled at all (your patch 6 only enables it for
> > ih_v6_0_irq_init, no equivalent in ih_v7_0_irq_init)
> 
> 
> For the attempt on Navi 48, I enabled the CAM in ih_v7_0 the same way I do
> for
 ih_v6_0 in the series. But, because I couldn't get it fully working, I
> didn't include any of that code in the series. 
> 
> > so retries fire
> > repeatedly on the IH ring instead of being deduped by CAM. That
> > matches what you're seeing .. amdgpu_vm_handle_fault keeps being
> > called but each call is on a fresh IRQ for the same address.
> > Two things that could be happening underneath:
> > - The fault handler runs but the updated PTE never reaches UTC L0 (TLB
> > invalidation gap). On NV4 we see this as "valid PTEs failing to
> > translate"
> > in our UMR captures.
> 
> 
> I think this explanation may fit what I saw.
> Why is it not reaching UTC L0?
> Also, how do you inspect this stuff in umr?
> 
> 
> > 2. What bits we check on src_data[2]:
> >
> >
> >
> > Honestly, we don't use src_data[2] for retry detection. We use it only
> > for
> > the cam_index: cam_index = entry->src_data[2] & 0x3ff;   /* low 10 bits =
> > CAM slot */
> 
> 
> 
> > For retry detection we initially used the gfx9 constant on src_data[1]
> > like you, but observed the bit cleared on a lot of NV4 events that
> > should have been retries (waves were hung in xnack-stall but no IH event
> > matched).
 So
> > we just go through the retry path unconditionally on NV4
> 
> 
> I see. I think I needed to change some gfxhub registers to get those
> src_data bits that you are missing.
 
> 
> > and let
> > amdgpu_vm_handle_fault sort it out via SVM range migration. May be
> > specific to gfx1201 / our test path
> 
> 
> My test case is a simple Vulkan shader which I am executing with vkrunner.
> Each shader invocation does an out of bounds read from a different page. For
> Navi 31 (and Strix Halo), I started out with just 1 page fault, and once I
> could mitigate that reliably, I turned it up to several hundred faults.
 
> 
> > 3. TLB flush making it worse .. clue about what to do:
> >
> >
> >
> > Honest answer: not really, not a SW-only fix. Our 1 GiB hang is an
> > architectural deadlock ... ih_soft_work blocks on a dma_fence for an
> > SDMA BO-clear, the BO-clear is stalled on a GCR (cache flush) request,
> > and the GC cache block isn't ACK'ing the GCR while UTC L2 is saturated
> > by the user shader's XNACK retry storm. Adding a TLB flush adds
> > another translation request to the same saturated UTC, which is why it
> > makes things worse.
> 
> 
> This may be related to a flaw in amdgpu_gmc_handle_retry_fault():
> what the function does is first call amdgpu_vm_handle_fault() which kicks
> off an SDMA job to update page tables, and then it ACKs the filter CAM.
> However, at the moment when the CAM is ACKed, the SDMA job is not finished
> yet, so the CP sees that the page is still invalid and the page fault
> interrupt is fired again. I actually noticed that on Navi 31 too, but it's
> just not fatal there.
 It just basically handles the same page fault twice.
> Once we solve this flaw, I would like to propose to enable retry faults by
> default on Navi 3. 
> Here is an idea for a solution:
> 
> Instead of ACKing the CAM right away, we should do it after the SDMA fence
> is signalled, ie. when we are sure the page tables are updated. Maybe we
> can set a callback on the fence and do it there, though it would require a
> slight code churn to get that to work.
 
> What do you think?
> 
> 
> > 4. IH1 ring on NV4:
> >
> >
> >
> > Same as you ... retry faults on NV4 always come in on IH0. We delegate
> > from
> > IH0 to ih.ring_soft (amdgpu_irq_delegate(adev, entry, 8)) so the
> > SVM/migration path can sleep, but the original entry is on IH0. We
> > haven't tried IH1 routing.
> 
> 
> Why, though?
> 
> The ih_v7_0 code does set up the IH1 ring and configure it exactly the same
> as the ih_v6_0, so I don't see why it wouldn't work?
 
> 
> > Re your branch: thanks for the gitlab link, easier than digging
> > through patchwork.
> > I'll cherry-pick patches 1, 3, 4 into our test build to see if patch 4
> > cleans up the timestamp filter delta we're seeing (97k entered / 2.8k
> > completed at 1 GiB might be partly explained by your Strix Halo bug).
> 
> 
> The timestamp issue actually gave me an endless headache when I first got
> into this topic in December. I hope the patch helps!
 
> Best regards,
> Timur
> 
>