linux-kernel - Re: [PATCH v1 5/8] iommufd: Add replace support in iommufd_access_set

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Message-ID: <Y9z+uAbYtgk30no+@nvidia.com>
Date:   Fri, 3 Feb 2023 08:31:52 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Nicolin Chen <nicolinc@...dia.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "will@...nel.org" <will@...nel.org>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>
Subject: Re: [PATCH v1 5/8] iommufd: Add replace support in
 iommufd_access_set_ioas()

On Fri, Feb 03, 2023 at 10:10:45AM +0000, Tian, Kevin wrote:
> > From: Nicolin Chen <nicolinc@...dia.com>
> > Sent: Thursday, February 2, 2023 3:05 PM
> > 
> > Support an access->ioas replacement in iommufd_access_set_ioas(), which
> > sets the access->ioas to NULL provisionally so that any further incoming
> > iommufd_access_pin_pages() callback can be blocked.
> > 
> > Then, call access->ops->unmap() to clean up the entire iopt. To allow an
> > iommufd_access_unpin_pages() callback to happen via this unmap() call,
> > add an ioas_unpin pointer so the unpin routine won't be affected by the
> > "access->ioas = NULL" trick above.
> > 
> > Also, a vdev without an ops->dma_unmap implementation cannot replace its
> > access->ioas pointer. So add an iommufd_access_ioas_is_attached() helper
> > to sanity that.
> > 
> 
> Presumably a driver which doesn't implement ops->dma_unmap shouldn't
> be allowed to do pin/unpin. But it could use vfio_dma_rw() to access an
> iova range. In the latter case I don't see why replace cannot work.
> 
> Probably what's required here is to deny !ops->dma_unmap in
> vfio_pin/unpin_pages then making here replace always allowed?

That makes sense

Jason