Re: [PATCH 0/2] iommu/amd: Do not request ACS when IOMMU is not going to be initialized

[Ankit Soni]

On Tue, Aug 11, 2026 at 04:36:17AM +0800, Rong Zhang wrote:
> The AMD IOMMU Initialization State Machine has the following state
> transition diagram (only the very first states are showed, and the
> `IOMMU_' prefix is omitted):
> 
>    START_STATE
>         |
>         v
> [0] detect_ivrs() --> NOT_FOUND
>         | ok
>         v
>   IVRS_DETECTED
>         |
>         v
> [1] amd_iommu_disabled? (amd_iommu=off) --> IOMMU_CMDLINE_DISABLED
>         | no
>         v
> [2] early_amd_iommu_init()
>         |
>         +-- [3] amd_iommu_detected? (!iommu=off && ...) -+
>         | yes                                            |   
>         +-- ... --> IOMMU_INIT_ERROR <-------------------+
>         | ok
>         v
> IOMMU_ACPI_FINISHED
>         |
>         v
>        ...
> 
> [0] always calls pci_request_acs() as long as there's a valid IVRS table
> and no Stoney Ridge graphics. This is not optimal as ACS is not required
> in an [amd_]iommu=off boot.
> 
> In a normal boot, ACS is requested due to amd_iommu_detect() requesting
> IVRS_DETECTED.
> 
>     pci_request_acs+0x9/0x18
>     iommu_go_to_state+0x106/0x1a20
>     amd_iommu_detect+0x1c/0x50
>     pci_iommu_alloc+0x26/0x40
>     mm_core_init+0xa/0x120
>     start_kernel+0x527/0x7a0
>     x86_64_start_reservations+0x24/0x30
>     x86_64_start_kernel+0xd1/0xe0
>     common_startup_64+0x13e/0x158
> 
> This is intended to ensure ACS is requested before the PCI core
> initialization, or else a !CONFIG_IRQ_REMAP, nointremap or intremap=off
> boot would be broken.
> 
> However, in an amd_iommu=off boot, the state machine still requests ACS
> at the exact same time, as amd_iommu_detect() has nothing to do with
> amd_iommu_disabled.
> 
> Even worse, in an iommu=off boot, though amd_iommu_detect() bails out
> early, ACS is still requested due to amd_iommu_prepare() requesting
> IOMMU_ACPI_FINISHED, which is called by irq_remapping_prepare() thanks
> to CONFIG_X86_LOCAL_APIC (always set on X86_64) and CONFIG_IRQ_REMAP
> (enabled by defconfig), unless nointremap or intremap=off is also passed
> to cmdline.
> 
>     pci_request_acs+0x9/0x18
>     iommu_go_to_state+0x106/0x1a20
>     amd_iommu_prepare+0x15/0x40
>     irq_remapping_prepare+0x43/0x60
>     enable_IR_x2apic+0x22/0x190
>     x86_64_probe_apic+0xa/0x50
>     apic_intr_mode_init+0x70/0xd0
>     x86_late_time_init+0x28/0x40
>     start_kernel+0x6f9/0x7a0
>     ...
> 
> In both cases, [2] is still gated due to the [1] or [3] check, so that
> IOMMU can be disabled per cmdline.
> 
> Technically, it makes no sense to detect IVRS at all in an
> [amd_]iommu=off boot or if IOMMU is not supported due to platform
> settings. This is probably why amd_iommu_detect() bails out before
> requesting IVRS_DETECTED. Apparently only bailing out there is not
> sufficient, and the bailing-out paths should really have been parts of
> the state machine.
> 
> Fix it by moving the bailing-out paths and [1] to the right place in the
> state machine (i.e., before [0]), and always requesting IVRS_DETECTED in
> amd_iommu_detect() to initialize the state machine early and properly.
> This doesn't change the IOMMU initialization/failure sequence since
> commit 9f81ca8d1fd6 ("iommu/amd: Don't call early_amd_iommu_init() when
> AMD IOMMU is disabled").
> 
> Signed-off-by: Rong Zhang <i@xxxxxxxx>
> ---
> Rong Zhang (2):
>       iommu/amd: Do not request ACS when IOMMU is not going to be initialized
>       iommu/amd: Remove ad-hoc checks that are never true

Hi,

Thanks for the cleanup, and the analysis of the ACS call paths matches what I see.
Two things worth a look before this goes in.

1. Patch 2: I don't think the !amd_iommu_detected check in
   early_amd_iommu_init() is unreachable. pci_iommu_alloc() returns early for
   xen_pv_domain() before calling amd_iommu_detect(), but Xen PV still sets

	x86_init.irqs.intr_mode_init = x86_64_probe_apic;

   so x86_64_probe_apic() -> enable_IR_x2apic() -> irq_remapping_prepare() ->
   amd_iommu_prepare() enters the state machine anyway, and none of the new
   IOMMU_START_STATE conditions are true. That check looks like
   the only thing currently stopping early_amd_iommu_init() from running
   there. Could you double-check that case before dropping it?

2. Patch 1 Fixes: 9f81ca8d1fd6 doesn't look right - it only moved the
   amd_iommu_disabled check ahead of early_amd_iommu_init() and left
   IOMMU_START_STATE and detect_ivrs() alone, so ACS was requested with
   amd_iommu=off both before and after it. The commit that actually put
   pci_request_acs() on the detect path is 1adb7d31b051 ("iommu/amd: Fix
   pci_request_acs() call-place"). I'd drop Fixes: and Cc: stable and post
   this as a cleanup.

-Ankit

> 
>  drivers/iommu/amd/init.c | 36 ++++++++++++++----------------------
>  1 file changed, 14 insertions(+), 22 deletions(-)
> ---
> base-commit: db2ddb87143519e20a95aa36c60b36107b736a58
> change-id: d58d0f31-amd-iommu-fix-acs-8f3d4aba8183
> 
> Thanks,
> Rong
>