linux-kernel - Re: [PATCH v2 1/2] iommu: Prevent RESV_DIRECT devices from blocking domains

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Date:   Fri, 21 Jul 2023 16:35:37 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Kevin Tian <kevin.tian@...el.com>, Joerg Roedel <joro@...tes.org>,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Nicolin Chen <nicolinc@...dia.com>, iommu@...ts.linux.dev,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] iommu: Prevent RESV_DIRECT devices from blocking
 domains

On Thu, Jul 13, 2023 at 12:32:47PM +0800, Lu Baolu wrote:
> The IOMMU_RESV_DIRECT flag indicates that a memory region must be mapped
> 1:1 at all times. This means that the region must always be accessible to
> the device, even if the device is attached to a blocking domain. This is
> equal to saying that IOMMU_RESV_DIRECT flag prevents devices from being
> attached to blocking domains.
> 
> This also implies that devices that implement RESV_DIRECT regions will be
> prevented from being assigned to user space since taking the DMA ownership
> immediately switches to a blocking domain.
> 
> The rule of preventing devices with the IOMMU_RESV_DIRECT regions from
> being assigned to user space has existed in the Intel IOMMU driver for
> a long time. Now, this rule is being lifted up to a general core rule,
> as other architectures like AMD and ARM also have RMRR-like reserved
> regions. This has been discussed in the community mailing list and refer
> to below link for more details.
> 
> Other places using unmanaged domains for kernel DMA must follow the
> iommu_get_resv_regions() and setup IOMMU_RESV_DIRECT - we do not restrict
> them in the core code.
> 
> Cc: Robin Murphy <robin.murphy@....com>
> Cc: Alex Williamson <alex.williamson@...hat.com>
> Cc: Kevin Tian <kevin.tian@...el.com>
> Signed-off-by: Jason Gunthorpe <jgg@...dia.com>
> Link: https://lore.kernel.org/linux-iommu/BN9PR11MB5276E84229B5BD952D78E9598C639@BN9PR11MB5276.namprd11.prod.outlook.com
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> ---
>  include/linux/iommu.h |  2 ++
>  drivers/iommu/iommu.c | 37 +++++++++++++++++++++++++++----------
>  2 files changed, 29 insertions(+), 10 deletions(-)

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

Jason