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

[Baolu Lu]

On 12/3/25 08:06, Jacob Pan wrote:
> Hi Baolu,
>
> On Tue, 2 Dec 2025 10:17:34 +0800
> Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx>
> > > ---
> > >    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);
}

Thanks,
baolu