linux-kernel - Re: [PATCH 5/7] iommu/arm-smmu-v3: Allow building as a module

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Message-ID: <20191108110310.GC19153@willie-the-truck>
Date:   Fri, 8 Nov 2019 11:03:10 +0000
From:   Will Deacon <will@...nel.org>
To:     Joerg Roedel <joro@...tes.org>
Cc:     iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Robin Murphy <robin.murphy@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Subject: Re: [PATCH 5/7] iommu/arm-smmu-v3: Allow building as a module

Hi Joerg,

On Tue, Nov 05, 2019 at 01:15:08PM +0100, Joerg Roedel wrote:
> On Thu, Oct 31, 2019 at 03:42:47PM +0000, Will Deacon wrote:
> > Generally, I think unloading the IOMMU driver module while there are
> > active users is a pretty bad idea, much like unbinding the driver via
> > /sys in the same situation would also be fairly daft. However, I *think*
> > the code in __device_release_driver() tries to deal with this by
> > iterating over the active consumers and ->remove()ing them first.
> > 
> > I'm without hardware access at the moment, so I haven't been able to
> > test this myself. We could nobble the module_exit() hook, but there's
> > still the "force unload" option depending on the .config.
> 
> Okay, but besides the force-unload case, can we prevent accidential
> unloading by taking a reference to the module in add_device() and release
> it in remove_device()?

That's probably a sensible starting point, yes. In conjunction with the
patch from Jean-Philippe to introduce bus_unset_iommu(), we might have
a fighting chance of getting this to work.

I'll spin a v2.

Thanks!

Will