[PATCH 2/2] drm/amdkfd: use cache GTT buffer for PQ and wb pool

[Liu, Monk]

[AMD Official Use Only - AMD Internal Distribution Only]

Christian

What do you mean "What happens instead is that the broken KVM patch applies the guest caching attributes to the ring buffer instead of the host ones."

From what I heard there was a KVM patch to correct the mapping behavior -- previously the mapping is forced to cache mode even guest KMD maps the buffer with "WC".
But after the patch the buffer will be really "WC" if guest maps it with WC"... and this reveals the bug and hit our issue.

Can you explain why USWC for ring buffer is safe, why it will not hit coherence issue, why you don't make all ring buffers with USWC even for amdgpu side if you really believe USWC is innocent ...

Thanks

Monk Liu | Cloud GPU & Virtualization | AMD

-----Original Message-----
From: Christian König <ckoenig.leichtzumerken at gmail.com>
Sent: Friday, November 8, 2024 10:09 PM
To: Lazar, Lijo <Lijo.Lazar at amd.com>; Liu, Monk <Monk.Liu at amd.com>; Koenig, Christian <Christian.Koenig at amd.com>; Alex Deucher <alexdeucher at gmail.com>; Zhao, Victor <Victor.Zhao at amd.com>
Cc: amd-gfx at lists.freedesktop.org; Yang, Philip <Philip.Yang at amd.com>; Kuehling, Felix <Felix.Kuehling at amd.com>
Subject: Re: [PATCH 2/2] drm/amdkfd: use cache GTT buffer for PQ and wb pool

Am 08.11.24 um 12:26 schrieb Lazar, Lijo:
> On 11/8/2024 4:29 PM, Liu, Monk wrote:
>> [AMD Official Use Only - AMD Internal Distribution Only]
>>
>> To be clear for the mb() approach: Even if we insert mb() in prior to amdgpu_ring_set_wptr(ring), GPU might still fetch stalled data from PQ due to USWC attributes.
>>
> Inserting an mb() doesn't cause WC buffer flush is a wrong assumption.
>
> All prior loads/stores are supposed to be globally visible. Hence mb()
> followed by a write pointer update also should guarantee the same
> (From Arch manual).
>
>       The MFENCE instruction establishes a memory fence for both loads and
> stores. The processor ensures that no load or store after MFENCE will
> become globally visible *until all loads and stores before MFENCE are
> globally visible.*
>
>
> Ignoring the amdgpu driver part of it - if mb() is not working as
> expected as you claim that means something is wrong with the system.
>
> USWC or WB for ring type may still be a separate discussion.

Yeah completely agree. Additional to that the combine behavior between USWC and WB is the same, so the argumentation is clearly not correct.

What happens instead is that the broken KVM patch applies the guest caching attributes to the ring buffer instead of the host ones.

Making an educated guess I would say that the root of the problem is that the guest is using some incorrect caching attributes for some reason, which is exactly the same reason why that KVM patch was revert upstream as well.

Regards,
Christian.

>
> Thanks,
> Lijo
>
>> The issue here is not the re-ordering but the stalled PQ.
>>
>> Monk Liu | Cloud GPU & Virtualization | AMD
>>
>>
>>
>> -----Original Message-----
>> From: Liu, Monk
>> Sent: Friday, November 8, 2024 6:22 PM
>> To: Koenig, Christian <Christian.Koenig at amd.com>; Lazar, Lijo
>> <Lijo.Lazar at amd.com>; Alex Deucher <alexdeucher at gmail.com>; Zhao,
>> Victor <Victor.Zhao at amd.com>
>> Cc: amd-gfx at lists.freedesktop.org; Yang, Philip
>> <Philip.Yang at amd.com>; Kuehling, Felix <Felix.Kuehling at amd.com>
>> Subject: RE: [PATCH 2/2] drm/amdkfd: use cache GTT buffer for PQ and
>> wb pool
>>
>> Christian/Lijo
>>
>> We verified all approaches, and we know what works and not works, obviously the mb() doesn't work.
>>
>> The way of mb() between set wptr_polling and kicking off doorbell is theoretically correct, and I'm not object to do so... but this won't resolve the issue we hit.
>> First of all, USWC will have some chance that the data is still in CPU's WC storage place and not flushed to the memory and even with MB() get rid of re-ordering GPU might still have a chance to read stalled data from ring buffer as long as it is mapped USWC.
>>
>> This is why only cache plus snoop memory can get rid of inconsistence issues.
>>
>> Besides, do you know what's the rational to keep all GFX KCQ cache+snoop but only HIQ/KIQ from KFD configured to USWC ?
>>
>> For performance concern that looks to me always the second priority compared to "correct" especially under the case HIQ contributes very little to ROCM performance when switching to cache mapping.
>>
>>
>> Monk Liu | Cloud GPU & Virtualization | AMD
>>
>>
>>
>>
>>
>>
>>
>> -----Original Message-----
>> From: Koenig, Christian <Christian.Koenig at amd.com>
>> Sent: Thursday, November 7, 2024 7:57 PM
>> To: Lazar, Lijo <Lijo.Lazar at amd.com>; Alex Deucher
>> <alexdeucher at gmail.com>; Zhao, Victor <Victor.Zhao at amd.com>
>> Cc: amd-gfx at lists.freedesktop.org; Liu, Monk <Monk.Liu at amd.com>;
>> Yang, Philip <Philip.Yang at amd.com>; Kuehling, Felix
>> <Felix.Kuehling at amd.com>
>> Subject: Re: [PATCH 2/2] drm/amdkfd: use cache GTT buffer for PQ and
>> wb pool
>>
>> Am 07.11.24 um 06:58 schrieb Lazar, Lijo:
>>> On 11/6/2024 8:42 PM, Alex Deucher wrote:
>>>> On Wed, Nov 6, 2024 at 1:49 AM Victor Zhao <Victor.Zhao at amd.com> wrote:
>>>>> From: Monk Liu <Monk.Liu at amd.com>
>>>>>
>>>>> As cache GTT buffer is snooped, this way the coherence between CPU
>>>>> write and GPU fetch is guaranteed, but original code uses WC +
>>>>> unsnooped for HIQ PQ(ring buffer) which introduces coherency issues:
>>>>> MEC fetches a stall data from PQ and leads to MEC hang.
>>>> Can you elaborate on this?  I can see CPU reads being slower
>>>> because the memory is uncached, but the ring buffer is mostly writes anyway.
>>>> IIRC, the driver uses USWC for most if not all of the other ring
>>>> buffers managed by the kernel.  Why aren't those a problem?
>>> We have this on other rings -
>>>           mb();
>>>           amdgpu_ring_set_wptr(ring);
>>>
>>> I think the solution should be to use barrier before write pointer
>>> updates rather than relying on PCIe snooping.
>> Yeah, completely agree as well. The barrier also takes care of preventing the compiler from re-ordering writes.
>>
>> Regards,
>> Christian.
>>
>>> Thanks,
>>> Lijo
>>>
>>>> Alex
>>>>
>>>>> Signed-off-by: Monk Liu <Monk.Liu at amd.com>
>>>>> ---
>>>>>    drivers/gpu/drm/amd/amdkfd/kfd_device.c | 2 +-
>>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_device.c
>>>>> b/drivers/gpu/drm/amd/amdkfd/kfd_device.c
>>>>> index 1f1d79ac5e6c..fb087a0ff5bc 100644
>>>>> --- a/drivers/gpu/drm/amd/amdkfd/kfd_device.c
>>>>> +++ b/drivers/gpu/drm/amd/amdkfd/kfd_device.c
>>>>> @@ -779,7 +779,7 @@ bool kgd2kfd_device_init(struct kfd_dev *kfd,
>>>>>           if (amdgpu_amdkfd_alloc_gtt_mem(
>>>>>                           kfd->adev, size, &kfd->gtt_mem,
>>>>>                           &kfd->gtt_start_gpu_addr, &kfd->gtt_start_cpu_ptr,
>>>>> -                       false, true)) {
>>>>> +                       false, false)) {
>>>>>                   dev_err(kfd_device, "Could not allocate %d bytes\n", size);
>>>>>                   goto alloc_gtt_mem_failure;
>>>>>           }
>>>>> --
>>>>> 2.34.1
>>>>>