linux-kernel - RE: [RFC 01/20] iommu/iommufd: Add /dev/iommu core

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Message-ID: <BN9PR11MB543342C7757E85E9C41733B48CA29@BN9PR11MB5433.namprd11.prod.outlook.com>
Date:   Wed, 22 Sep 2021 13:59:39 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     "Liu, Yi L" <yi.l.liu@...el.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
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Subject: RE: [RFC 01/20] iommu/iommufd: Add /dev/iommu core

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Wednesday, September 22, 2021 8:41 PM
> 
> On Wed, Sep 22, 2021 at 01:51:03AM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@...dia.com>
> > > Sent: Tuesday, September 21, 2021 11:42 PM
> > >
> > >  - Delete the iommufd_ctx->lock. Use RCU to protect load, erase/alloc
> does
> > >    not need locking (order it properly too, it is in the wrong order), and
> > >    don't check for duplicate devices or dev_cookie duplication, that
> > >    is user error and is harmless to the kernel.
> > >
> >
> > I'm confused here. yes it's user error, but we check so many user errors
> > and then return -EINVAL, -EBUSY, etc. Why is this one special?
> 
> Because it is expensive to calculate and forces a complicated locking
> scheme into the kernel. Without this check you don't need the locking
> that spans so much code, and simple RCU becomes acceptable.
> 

In case of duplication the kernel just uses the first entry which matches
the device when sending an event to userspace?