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Date:   Mon, 24 Oct 2022 13:53:36 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Niklas Schnelle <schnelle@linux.ibm.com>
Cc:     iommu@lists.linux.dev, Joerg Roedel <joro@8bytes.org>,
        Will Deacon <will@kernel.org>,
        Robin Murphy <robin.murphy@arm.com>,
        Matthew Rosato <mjrosato@linux.ibm.com>,
        Gerd Bayer <gbayer@linux.ibm.com>,
        Pierre Morel <pmorel@linux.ibm.com>,
        linux-s390@vger.kernel.org, borntraeger@linux.ibm.com,
        hca@linux.ibm.com, gor@linux.ibm.com,
        gerald.schaefer@linux.ibm.com, agordeev@linux.ibm.com,
        svens@linux.ibm.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v7 3/6] iommu/s390: Fix potential s390_domain aperture
 shrinking
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References: <20221017124558.1386337-1-schnelle@linux.ibm.com>
 <20221017124558.1386337-4-schnelle@linux.ibm.com>
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On Mon, Oct 17, 2022 at 02:45:55PM +0200, Niklas Schnelle wrote:
> The s390 IOMMU driver currently sets the IOMMU domain's aperture to
> match the device specific DMA address range of the device that is first
> attached. This is not ideal. For one if the domain has no device
> attached in the meantime the aperture could be shrunk allowing
> translations outside the aperture to exist in the translation tables.
> Also this is a bit of a misuse of the aperture which really should
> describe what addresses can be translated and not some device specific
> limitations.
> 
> Instead of misusing the aperture like this we can instead create
> reserved ranges for the ranges inaccessible to the attached devices
> allowing devices with overlapping ranges to still share an IOMMU domain.
> This also significantly simplifies s390_iommu_attach_device() allowing
> us to move the aperture check to the beginning of the function and
> removing the need to hold the device list's lock to check the aperture.
> 
> As we then use the same aperture for all domains and it only depends on
> the table properties we can already check zdev->start_dma/end_dma at
> probe time and turn the check on attach into a WARN_ON().
> 
> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
> Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
> ---
> v5->v6:
> - Return -EINVAL after WARN_ON() in attach
> v4->v5:
> - Make aperture check in attach a WARN_ON() and fail in probe if
>   zdev->start_dma/end_dma doesn't git in aperture  (Jason)

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

Jason