linux-kernel - Re: [RFC 2/8] iommu: Add a helper to check if any iommu device is registered

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Message-ID: <20251203142835.00002a0c@linux.microsoft.com>
Date: Wed, 3 Dec 2025 14:28:35 -0800
From: Jacob Pan <jacob.pan@...ux.microsoft.com>
To: Baolu Lu <baolu.lu@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org, "iommu@...ts.linux.dev"
 <iommu@...ts.linux.dev>, Jason Gunthorpe <jgg@...dia.com>, Alex Williamson
 <alex.williamson@...hat.com>, Joerg Roedel <joro@...tes.org>, Will Deacon
 <will@...nel.org>, Robin Murphy <robin.murphy@....com>, Nicolin Chen
 <nicolinc@...dia.com>, "Tian, Kevin" <kevin.tian@...el.com>, "Liu, Yi L"
 <yi.l.liu@...el.com>, skhawaja@...gle.com, pasha.tatashin@...een.com, Zhang
 Yu <zhangyu1@...ux.microsoft.com>, Jean Philippe-Brucker
 <jean-philippe@...aro.org>, David Matlack <dmatlack@...gle.com>, Alex
 Williamson <alex@...zbot.org>
Subject: Re: [RFC 2/8] iommu: Add a helper to check if any iommu device is
 registered

Hi Baolu,

On Wed, 3 Dec 2025 11:31:27 +0800
Baolu Lu <baolu.lu@...ux.intel.com> wrote:

> On 12/3/25 08:06, Jacob Pan wrote:
> > Hi Baolu,
> > 
> > On Tue, 2 Dec 2025 10:17:34 +0800
> > Baolu Lu <baolu.lu@...ux.intel.com> wrote:
> >   
> >> Hi Jacob,
> >>
> >> On 12/2/25 01:30, Jacob Pan wrote:  
> >>> The dummy IOMMU driver for No-IOMMU mode should only be active
> >>> when no real IOMMU devices are present in the system. Introduce a
> >>> helper to check this condition, ensuring that the dummy driver
> >>> does not interfere when hardware-backed IOMMU support is
> >>> available.
> >>>
> >>> Signed-off-by: Jacob Pan<jacob.pan@...ux.microsoft.com>
> >>> ---
> >>>    drivers/iommu/iommu.c | 10 ++++++++++
> >>>    include/linux/iommu.h |  1 +
> >>>    2 files changed, 11 insertions(+)
> >>>
> >>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> >>> index 0df914a04064..958f612bf176 100644
> >>> --- a/drivers/iommu/iommu.c
> >>> +++ b/drivers/iommu/iommu.c
> >>> @@ -2895,6 +2895,16 @@ static const struct iommu_device
> >>> *iommu_from_fwnode(const struct fwnode_handle * return ret;
> >>>    }
> >>>    
> >>> +bool iommu_is_registered(void)
> >>> +{
> >>> +	bool registered;
> >>> +
> >>> +	spin_lock(&iommu_device_lock);
> >>> +	registered = !list_empty(&iommu_device_list);
> >>> +	spin_unlock(&iommu_device_lock);
> >>> +	return registered;
> >>> +}  
> >>
> >> IOMMU devices might be added by calling iommu_device_register() at
> >> any time. Therefore, an empty iommu_device_list does not
> >> necessarily mean that "no real IOMMU devices are present in the
> >> system."  
> > Good point. My intention was that the noiommu dummy driver should
> > register only after all hardware IOMMU drivers have completed
> > registration during boot. Any subsequent registration attempt, such
> > as a hot-added IOMMU, should fail if noiommu mode is already active.
> > 
> > We could enforce this by introducing a global flag that prevents any
> > iommu_device from being registered after the noiommu driver has been
> > initialized.
> > 
> > However, as you pointed out there seems to be no standard ordering
> > for iommu device registration across platforms. e.g. VT-d hooks up
> > with x86_init, smmuv3 does that in platform driver probe. This
> > patchset puts dummy driver under early_initcall which is after both
> > but not a guarantee for all platforms. Any suggestions?  
> 
> Could we add a helper call inside the IOMMU detection path (e.g.,
> within pci_iommu_alloc()) to set a flag, such as
> platform_iommu_present, after successful hardware detection?
> 
> void __init pci_iommu_alloc(void)
> {
>          if (xen_pv_domain()) {
>                  pci_xen_swiotlb_init();
>                  return;
>          }
>          pci_swiotlb_detect();
>          gart_iommu_hole_init();
>          amd_iommu_detect();
>          detect_intel_iommu();
>          swiotlb_init(x86_swiotlb_enable, x86_swiotlb_flags);
> }
I think this would only work for x86.