linux-kernel - Re: [PATCH v3 1/2] vfio/type1: Simplify bus

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Date:   Fri, 24 Jun 2022 15:50:02 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     alex.williamson@...hat.com, cohuck@...hat.com, kvm@...r.kernel.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        baolu.lu@...ux.intel.com, iommu@...ts.linux.dev
Subject: Re: [PATCH v3 1/2] vfio/type1: Simplify bus_type determination

On Fri, Jun 24, 2022 at 06:51:44PM +0100, Robin Murphy wrote:
> Since IOMMU groups are mandatory for drivers to support, it stands to
> reason that any device which has been successfully added to a group
> must be on a bus supported by that IOMMU driver, and therefore a domain
> viable for any device in the group must be viable for all devices in
> the group. This already has to be the case for the IOMMU API's internal
> default domain, for instance. Thus even if the group contains devices on
> different buses, that can only mean that the IOMMU driver actually
> supports such an odd topology, and so without loss of generality we can
> expect the bus type of any device in a group to be suitable for IOMMU
> API calls.
> 
> Furthermore, scrutiny reveals a lack of protection for the bus being
> removed while vfio_iommu_type1_attach_group() is using it; the reference
> that VFIO holds on the iommu_group ensures that data remains valid, but
> does not prevent the group's membership changing underfoot.
> 
> We can address both concerns by recycling vfio_bus_type() into some
> superficially similar logic to indirect the IOMMU API calls themselves.
> Each call is thus protected from races by the IOMMU group's own locking,
> and we no longer need to hold group-derived pointers beyond that scope.
> It also gives us an easy path for the IOMMU API's migration of bus-based
> interfaces to device-based, of which we can already take the first step
> with device_iommu_capable(). As with domains, any capability must in
> practice be consistent for devices in a given group - and after all it's
> still the same capability which was expected to be consistent across an
> entire bus! - so there's no need for any complicated validation.
> 
> Signed-off-by: Robin Murphy <robin.murphy@....com>
> ---
> 
> v3: Complete rewrite yet again, and finally it doesn't feel like we're
> stretching any abstraction boundaries the wrong way, and the diffstat
> looks right too. I did think about embedding IOMMU_CAP_INTR_REMAP
> directly in the callback, but decided I like the consistency of minimal
> generic wrappers. And yes, if the capability isn't supported then it
> does end up getting tested for the whole group, but meh, it's harmless.

I think this is really weird, but it works and isn't worth debating
further, so OK:

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason