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

[Ankit Soni]

On Thu, Aug 20, 2026 at 01:38:35AM +0800, Rong Zhang wrote:
> Hi Ankit,
> 
> Thanks for your review.
> 
> On Wed, 2026-08-19 at 10:32 +0000, Ankit Soni wrote:
> > 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
> > >        ...
> > > 

../..

> > > ---
> > > 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?
> 
> This has been resolved by PATCH 1, where the checks have been embedded
> into the state machine.

Agreed on patch 1 - the state machine is the right place for those checks and
I have no objection to it. My concern is only patch 2 dropping the
"amd_iommu_detected" check.

> 
> I did some quick tests by commenting out the call to amd_iommu_detect()
> in pci_iommu_alloc(), and adding a debug WARN() to iommu_go_to_state().
> When I booted a VM with emulated IOMMU (-device amd-iommu) and with
> either `amd_iommu=off' or `iommu=off':
> 
> [    0.081948] APIC: Switch to symmetric I/O mode setup
> [    0.082553] ------------[ cut here ]------------
> [    0.083144] init_state: 0 => 9, ret: -22
> [    0.083617] WARNING: drivers/iommu/amd/init.c:3506 at iommu_go_to_state+0xf18/0x1c00, CPU#0: swapper/0/0
> [    0.084809] Modules linked in:
> [    0.085122] CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 7.2.0-02579-ge0f12714fa53-dirty #38 PREEMPT(lazy)
> [    0.086124] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.17.0-debian-1.17.0-1 04/01/2014
> [    0.087076] RIP: 0010:iommu_go_to_state+0xf23/0x1c00
> [    0.087629] Code: 1b e7 2c 00 eb 12 c7 05 0f e7 2c 00 06 00 00 00 eb 03 90 0f 0b 45 31 f6 48 8d 3d e8 90 b6 ff 8b 15 fa e6 2c 00 44 89 f1 89 de <67> 48 0f b9 3a 45 85 f6 0f 84 f4 f0 ff ff e8 8a c0 73 fe 48 8b 05
> [    0.089556] RSP: 0000:ffffffff93a03e40 EFLAGS: 00010202
> [    0.090039] RAX: 00000000fffffff9 RBX: 0000000000000000 RCX: 00000000ffffffea
> [    0.090677] RDX: 0000000000000009 RSI: 0000000000000000 RDI: ffffffff93d17440
> [    0.091338] RBP: ffff8fbd47fd5c80 R08: 3fffffffffffdfff R09: ffffffff93b20aa8
> [    0.092025] R10: 0000000000000002 R11: 0000000000000002 R12: 000000000000001b
> [    0.092662] R13: 0000000000000000 R14: 00000000ffffffea R15: 0000000000014770
> [    0.093319] FS:  0000000000000000(0000) GS:ffff8fbdb3634000(0000) knlGS:0000000000000000
> [    0.094055] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [    0.094572] CR2: ffff8fbd4728f000 CR3: 0000000006844000 CR4: 00000000000100b0
> [    0.095214] Call Trace:
> [    0.095437]  <TASK>
> [    0.095628]  amd_iommu_prepare+0x15/0x40
> [    0.096025]  irq_remapping_prepare+0x43/0x60
> [    0.096406]  enable_IR_x2apic+0x22/0x190
> [    0.096759]  x86_64_probe_apic+0xa/0x50
> [    0.097130]  apic_intr_mode_init+0x70/0xd0
> [    0.097495]  x86_late_time_init+0x28/0x40
> [    0.097858]  start_kernel+0x6f9/0x7a0
> [    0.098209]  x86_64_start_reservations+0x24/0x30
> [    0.098629]  x86_64_start_kernel+0xd1/0xe0
> [    0.099016]  common_startup_64+0x13e/0x158
> [    0.099393]  </TASK>
> [    0.099595] ---[ end trace 0000000000000000 ]---
> [    0.100266] x2apic enabled
> [    0.100758] APIC: Switched APIC routing to: physical x2apic
> 
> ...where 0 is IOMMU_START_STATE and 9 is IOMMU_CMDLINE_DISABLED.

Commenting out amd_iommu_detect() is right way to emulate this, and the trace 
confirms the entry path. But the boot options are what make it
come out clean: amd_iommu=off / iommu=off set amd_iommu_disabled / no_iommu,
so IOMMU_START_STATE bails on its first if - that's the 0 => 9., which
is clearly visible above.

But,
Xen's own iommu= sits on the hypervisor command line and goes to Xen's parser,
not to Linux's iommu_setup(), and there no one sets no_iommu or amd_iommu_disabled.

The wider point is that amd_iommu_detect() never running is the platform mean
Linux does not own this IOMMU - Xen opts out of pci_iommu_alloc() and installs
xen_swiotlb_dma_ops instead. 'amd_iommu_detected' was the only thing making the
irq-remapping entry path honour that, so without it (i.e. patch 2 removes it) 
early_amd_iommu_init() proceeds on hardware the hypervisor owns.

-Ankit

> 
> Therefore, embedding the checks into the state machine ensures the checks
> are done in all initialization paths and makes replicating the same
> checks in state transition methods unnecessary.
> 
> > 
> > 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.
> 
> Makes sense. Will drop them when resubmitting. I will defer to your
> opinion on my explanation to the first question before resubmitting :)
> 
> Thanks,
> Rong
> 
> > 
> > -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
> > >