RE: [PATCH 1/1] iommu/vt-d: Revert ATS timing change to fix boot failure

[Tian, Kevin]

> From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> Sent: Wednesday, April 16, 2025 3:36 PM
> 
> Commit <5518f239aff1> ("iommu/vt-d: Move scalable mode ATS enablement
> to
> probe path") changed the PCI ATS enablement logic to run earlier,
> specifically before the default domain attachment.
> 
> On some client platforms, this change resulted in boot failures, causing
> the kernel to panic with the following message and call trace:
> 
>  Kernel panic - not syncing: DMAR hardware is malfunctioning
>  CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.14.0-rc3+ #175
>  Call Trace:
>   <TASK>
>   dump_stack_lvl+0x6f/0xb0
>   dump_stack+0x10/0x16
>   panic+0x10a/0x2b7
>   iommu_enable_translation.cold+0xc/0xc
>   intel_iommu_init+0xe39/0xec0
>   ? trace_hardirqs_on+0x1e/0xd0
>   ? __pfx_pci_iommu_init+0x10/0x10
>   pci_iommu_init+0xd/0x40
>   do_one_initcall+0x5b/0x390
>   kernel_init_freeable+0x26d/0x2b0
>   ? __pfx_kernel_init+0x10/0x10
>   kernel_init+0x15/0x120
>   ret_from_fork+0x35/0x60
>   ? __pfx_kernel_init+0x10/0x10
>   ret_from_fork_asm+0x1a/0x30
>  RIP: 1f0f:0x0
>  Code: Unable to access opcode bytes at 0xffffffffffffffd6.
>  RSP: 0000:0000000000000000 EFLAGS: 841f0f2e66 ORIG_RAX:
>       1f0f2e6600000000
>  RAX: 0000000000000000 RBX: 1f0f2e6600000000 RCX:
>       2e66000000000084
>  RDX: 0000000000841f0f RSI: 000000841f0f2e66 RDI:
>       00841f0f2e660000
>  RBP: 00841f0f2e660000 R08: 00841f0f2e660000 R09:
>       000000841f0f2e66
>  R10: 0000000000841f0f R11: 2e66000000000084 R12:
>       000000841f0f2e66
>  R13: 0000000000841f0f R14: 2e66000000000084 R15:
>       1f0f2e6600000000
>   </TASK>
>  ---[ end Kernel panic - not syncing: DMAR hardware is malfunctioning ]---
> 
> Fix this by reverting the timing change for ATS enablement introduced by
> the offending commit and restoring the previous behavior.
> 

it's unclear how this timing is related to the dumped stack. Is there
more detail how they are related?