Re: [REGRESSION] iommu/amd: "Fix premature break in init_iommu_one()" causes early boot hang

Hi Vasant,


On 8/24/26 07:52, Vasant Hegde wrote:
> Hl Andreas,
>
>
> On 8/22/2026 7:02 PM, Andreas Juch wrote:
>> [Some people who received this message don't often get email from
>> [email protected]. Learn why this is important at https://aka.ms/
>> LearnAboutSenderIdentification ]
>>
>> Hello Vasant,
>>
>> On 8/22/26 12:17, Vasant Hegde wrote:
>>> Andreas,
>>>
>>> On 8/22/2026 11:19 AM, Joerg Roedel wrote:
>>>> Adding Vasant.
>>>>
>>>> On Fri, Aug 21, 2026 at 11:11:07PM +0200, Andreas Juch wrote:
>>>>> Hello,
>>>>>
>>>>> I noticed that one of my NixOS sytems failed to boot with kernel 6.18.40.
>>>>> The
>>>>> older 6.18.39 booted fine. A full git bisect of v6.18.39..v6.18.40
>>>>> identified
>>>>> the following commit as the first bad commit:
>>>>>
>>>>>     dc266f6c4e2628576ab33042b6b2a12480220026
>>>>>     iommu/amd: Fix premature break in init_iommu_one()
>>>>>
>>>>> The visible symptom is a black screen immediately after systemd-boot. The
>>>>> machine also remains unreachable over SSH and leaves no persistent journal
>>>>> entry
>>>>> for the failed boot. A reset or power cycle is required.
>>> Interesting. I never imagined above fix will break some system!
>>>
>>> I assumed below dmesg is from good boot (without above patch) and did the
>>> analysis (code walk with below IVRS data):
>> Yes, that's correct, it's from the .40 kernel without commit dc266f6c.
>>
>>>     - EFR advertised by IVRS is different from actual EFR (read by IOMMU MMIO
>>> register). Its worse because IVRS advertised few features like GT/PPR which is
>>> not supported by IOMMU.
>>>     - Without this patch we would have skipped early_iommu_features_init() so
>>> global EFR is zero and then init_iommu_one_late() set the EFR properly based on
>>> actual IOMMU capability.
>>>
>>> Only with this log, its hard to tell where exactly system failed. My guess is
>>> driver enabled features which are not supported (like GT/PPR) or we setup 32bit
>>> interrupt remapping table.
>>>
>>> I believe above patch exposed an existing bug with some other commit.
>>>
>>>
>>> Ideally we should properly fix late_iommu_features_init() and use actual EFRs.
>>> But for now lets fix the regression. Can you please try below fix?
>>>
>>> -Vasant
>> Thank you for investigating! I created a new branch from the v6.18.40
>> tag and
>> applied your patch. The resulting kernel booted without any issues!
>
> Thanks a lot. This is like another band-aid fix, but I think it's good enough
> for a stable backport. In my view, we should prioritize the IOMMU
> MMIO-advertised EFR over the one reported via IVRS. However, that would require
> much more extensive changes and isn't a good candidate for stable.
>
>
>> If you need any further information or testing (for a proper fix later
>> on), I'd
>> be happy to test patches again!
> Can you help me to test below patch (just apply this patch and remove previously
> provided fix patch). I want to make sure that prioritizing MMIO EFR doesn't
> break things.
I compiled the kernel with just your patch on top of the 6.18.40 tag and it
didn't boot. Please let me know how I can provide better diagnostics. I 
tried to
get serial output over IPMI SoL, but never got anything useful back.


Andreas

> -Vasant
>
> ---<---
> commit aa5f50f1ee66ebf7a219906a4df6aa2b7cae9717
> Author: Vasant Hegde <[email protected]>
> Date:   Mon Aug 24 11:12:27 2026 +0530
>
>      iommu/amd: Prioritize IOMMU MMIO advertised EFR over IVRS
>
>      Signed-off-by: Vasant Hegde <[email protected]>
>
> diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
> index 69170146d442..0172db87b1df 100644
> --- a/drivers/iommu/amd/init.c
> +++ b/drivers/iommu/amd/init.c
> @@ -2153,9 +2153,11 @@ static void __init late_iommu_features_init(struct
> amd_iommu *iommu)
>   	if (features != amd_iommu_efr ||
>   	    features2 != amd_iommu_efr2) {
>   		pr_warn(FW_WARN
> -			"EFR mismatch. Use IVHD EFR (%#llx : %#llx), EFR2 (%#llx : %#llx).\n",
> +			"EFR mismatch. Use MMIO EFR (%#llx : %#llx), EFR2 (%#llx : %#llx).\n",
>   			features, amd_iommu_efr,
>   			features2, amd_iommu_efr2);
> +		amd_iommu_efr = features;
> +		amd_iommu_efr2 = features2;
>   	}
>   }
>
>
>
>
>
>
>> Thanks again!
>>
>> Andreas
>>
>>> ---<---
>>>
>>> diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
>>> index 69170146d442..7a5b78beb352 100644
>>> --- a/drivers/iommu/amd/init.c
>>> +++ b/drivers/iommu/amd/init.c
>>> @@ -1930,19 +1930,20 @@ static int __init init_iommu_one(struct amd_iommu
>>> *iommu, struct ivhd_header *h,
>>>                else
>>>                        iommu->mmio_phys_end = MMIO_CNTR_CONF_OFFSET;
>>>
>>> +             if (h->efr_attr & BIT(IOMMU_IVHD_ATTR_HATDIS_SHIFT)) {
>>> +                     pr_warn_once("Host Address Translation is not supported.
>>> \n");
>>> +                     amd_iommu_hatdis = true;
>>> +             }
>>> +
>>>                /* XT and GAM require GA mode. */
>>>                if ((h->efr_reg & (0x1 << IOMMU_EFR_GASUP_SHIFT)) == 0) {
>>>                        amd_iommu_guest_ir = AMD_IOMMU_GUEST_IR_LEGACY;
>>> +                     break;
>>>                } else {
>>>                        if (h->efr_reg & BIT(IOMMU_EFR_XTSUP_SHIFT))
>>>                                amd_iommu_xt_mode = IRQ_REMAP_X2APIC_MODE;
>>>                }
>>>
>>> -             if (h->efr_attr & BIT(IOMMU_IVHD_ATTR_HATDIS_SHIFT)) {
>>> -                     pr_warn_once("Host Address Translation is not supported.
>>> \n");
>>> -                     amd_iommu_hatdis = true;
>>> -             }
>>> -
>>>                early_iommu_features_init(iommu, h);
>>>
>>>                break;
>>>
>>>
>>>
>>>>> I reverted that single commit, built the kernel again and the machine
>>>>> booted, so
>>>>> I'm quite sure that I found the right commit.
>>>>>
>>>>> System information:
>>>>>
>>>>> ASRockRack B550D4-4L
>>>>> BIOS P1.10 (06/28/2021)
>>>>>
>>>>> I'm no expert at all, but my research found that the following information
>>>>> might
>>>>> be relevant:
>>>>>
>>>>>     journalctl -W -b -k --no-pager | rg -i 'AMD-Vi|AMD IOMMU|IOMMU|IVRS|IVHD'
>>>>>     Aug 21 22:24:17 kernel: ACPI: IVRS 0x00000000BB062000 0000D0 (v02 AMD
>>>>> AmdTable 00000001 AMD  00000001)
>>>>>     Aug 21 22:24:17 kernel: ACPI: Reserving IVRS table memory at [mem
>>>>> 0xbb062000-0xbb0620cf]
>>>>>     Aug 21 22:24:17 kernel: AMD-Vi: Using global IVHD EFR:0x0, EFR2:0x0
>>>>>     Aug 21 22:24:17 kernel: iommu: Default domain type: Translated
>>>>>     Aug 21 22:24:17 kernel: iommu: DMA domain TLB invalidation policy: lazy
>>>>> mode
>>>>>     Aug 21 22:24:17 kernel: pci 0000:00:01.0: Adding to iommu group 0
>>>>>     Aug 21 22:24:17 kernel: pci 0000:00:01.2: Adding to iommu group 0
>>>>>     Aug 21 22:24:17 kernel: pci 0000:00:02.0: Adding to iommu group 1
>>>>>     Aug 21 22:24:17 kernel: pci 0000:00:03.0: Adding to iommu group 2
>>>>>     Aug 21 22:24:17 kernel: pci 0000:00:04.0: Adding to iommu group 3
>>>>>     Aug 21 22:24:17 kernel: pci 0000:00:05.0: Adding to iommu group 4
>>>>>     Aug 21 22:24:17 kernel: pci 0000:00:07.0: Adding to iommu group 5
>>>>>     Aug 21 22:24:17 kernel: pci 0000:00:07.1: Adding to iommu group 6
>>>>>     Aug 21 22:24:17 kernel: pci 0000:00:08.0: Adding to iommu group 7
>>>>>     Aug 21 22:24:17 kernel: pci 0000:00:08.1: Adding to iommu group 8
>>>>>     Aug 21 22:24:17 kernel: pci 0000:00:14.0: Adding to iommu group 9
>>>>>     Aug 21 22:24:17 kernel: pci 0000:00:14.3: Adding to iommu group 9
>>>>>     Aug 21 22:24:17 kernel: pci 0000:00:18.0: Adding to iommu group 10
>>>>>     Aug 21 22:24:17 kernel: pci 0000:00:18.1: Adding to iommu group 10
>>>>>     Aug 21 22:24:17 kernel: pci 0000:00:18.2: Adding to iommu group 10
>>>>>     Aug 21 22:24:17 kernel: pci 0000:00:18.3: Adding to iommu group 10
>>>>>     Aug 21 22:24:17 kernel: pci 0000:00:18.4: Adding to iommu group 10
>>>>>     Aug 21 22:24:17 kernel: pci 0000:00:18.5: Adding to iommu group 10
>>>>>     Aug 21 22:24:17 kernel: pci 0000:00:18.6: Adding to iommu group 10
>>>>>     Aug 21 22:24:17 kernel: pci 0000:00:18.7: Adding to iommu group 10
>>>>>     Aug 21 22:24:17 kernel: pci 0000:02:00.0: Adding to iommu group 0
>>>>>     Aug 21 22:24:17 kernel: pci 0000:02:00.1: Adding to iommu group 0
>>>>>     Aug 21 22:24:17 kernel: pci 0000:02:00.2: Adding to iommu group 0
>>>>>     Aug 21 22:24:17 kernel: pci 0000:20:00.0: Adding to iommu group 0
>>>>>     Aug 21 22:24:17 kernel: pci 0000:20:01.0: Adding to iommu group 0
>>>>>     Aug 21 22:24:17 kernel: pci 0000:20:02.0: Adding to iommu group 0
>>>>>     Aug 21 22:24:17 kernel: pci 0000:20:03.0: Adding to iommu group 0
>>>>>     Aug 21 22:24:17 kernel: pci 0000:20:04.0: Adding to iommu group 0
>>>>>     Aug 21 22:24:17 kernel: pci 0000:20:08.0: Adding to iommu group 0
>>>>>     Aug 21 22:24:17 kernel: pci 0000:20:09.0: Adding to iommu group 0
>>>>>     Aug 21 22:24:17 kernel: pci 0000:21:00.0: Adding to iommu group 0
>>>>>     Aug 21 22:24:17 kernel: pci 0000:22:00.0: Adding to iommu group 0
>>>>>     Aug 21 22:24:17 kernel: pci 0000:23:00.0: Adding to iommu group 0
>>>>>     Aug 21 22:24:17 kernel: pci 0000:24:00.0: Adding to iommu group 0
>>>>>     Aug 21 22:24:17 kernel: pci 0000:25:00.0: Adding to iommu group 0
>>>>>     Aug 21 22:24:17 kernel: pci 0000:29:00.0: Adding to iommu group 0
>>>>>     Aug 21 22:24:17 kernel: pci 0000:2a:00.0: Adding to iommu group 0
>>>>>     Aug 21 22:24:17 kernel: pci 0000:2b:00.0: Adding to iommu group 0
>>>>>     Aug 21 22:24:17 kernel: pci 0000:2c:00.0: Adding to iommu group 11
>>>>>     Aug 21 22:24:17 kernel: pci 0000:2d:00.0: Adding to iommu group 12
>>>>>     Aug 21 22:24:17 kernel: pci 0000:2d:00.1: Adding to iommu group 13
>>>>>     Aug 21 22:24:17 kernel: pci 0000:2d:00.3: Adding to iommu group 14
>>>>>     Aug 21 22:24:17 kernel: pci 0000:2d:00.4: Adding to iommu group 15
>>>>>     Aug 21 22:24:17 kernel: AMD-Vi: Extended features (0x58f77e022294881,
>>>>> 0x0): PreF GA GA_vAPIC
>>>>>     Aug 21 22:24:17 kernel: AMD-Vi: Interrupt remapping enabled
>>>>>
>>>>> And the output of the decoded IVRS table, captured by `acpidump -n IVRS -b`:
>>>>>
>>>>>     00000000  49 56 52 53 d0 00 00 00  02 e2 41 4d 44 20 20 00 |IVRS......AMD
>>>>> .|
>>>>>     00000010  41 6d 64 54 61 62 6c 65  01 00 00 00 41 4d 44 20
>>>>> |AmdTable....AMD |
>>>>>     00000020  01 00 00 00 41 30 20 00  00 00 00 00 00 00 00 00 |....A0
>>>>> .........|
>>>>>     00000030  10 b0 48 00 02 00 40 00  00 00 50 fd 00 00 00 00
>>>>> |[email protected].....|
>>>>>     00000040  00 00 00 00 2e 8f 04 80  03 08 00 00 04 fe ff 00
>>>>> |................|
>>>>>     00000050  43 00 ff 00 00 a5 00 00  04 ff ff 00 00 00 00 00
>>>>> |C...............|
>>>>>     00000060  48 00 00 00 00 a0 00 02  48 00 00 d7 0d a0 00 01
>>>>> |H.......H.......|
>>>>>     00000070  48 00 00 00 0e 01 00 01  11 b0 58 00 02 00 40 00
>>>>> |H.........X...@.|
>>>>>     00000080  00 00 50 fd 00 00 00 00  00 00 00 00 00 02 04 00
>>>>> |..P.............|
>>>>>     00000090  5a 4a 29 22 ef 77 8f 05  00 00 00 00 00 00 00 00
>>>>> |ZJ)".w..........|
>>>>>     000000a0  03 08 00 00 04 fe ff 00  43 00 ff 00 00 a5 00 00
>>>>> |........C.......|
>>>>>     000000b0  04 ff ff 00 00 00 00 00  48 00 00 00 00 a0 00 02
>>>>> |........H.......|
>>>>>     000000c0  48 00 00 d7 0d a0 00 01  48 00 00 00 0e 01 00 01
>>>>> |H.......H.......|
>>>>>     000000d0
>>>>>
>>>>> The relevant part (after decoding with `iasl -d ivrs.dat`):
>>>>>
>>>>>     [090h 0144 008h]                   EFR Image : 058F77EF22294A5A
>>>>>
>>>>> I'll provide any required details and test patches!
>>>>>
>>>>> Thanks for your work, it's very appreciated!
>>>>>
>>>>> Andreas
>>>>>