linux-kernel - Re: [PATCH 2/5] iommu/vt-d: Remove WO permissions on second-level paging entries

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Message-ID: <20210304122623.GD26414@8bytes.org>
Date:   Thu, 4 Mar 2021 13:26:23 +0100
From:   Joerg Roedel <joro@...tes.org>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Will Deacon <will@...nel.org>, ashok.raj@...el.com,
        kevin.tian@...el.com, jacob.jun.pan@...el.com, yi.l.liu@...el.com,
        sanjay.k.kumar@...el.com, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] iommu/vt-d: Remove WO permissions on second-level
 paging entries

On Thu, Feb 25, 2021 at 02:26:51PM +0800, Lu Baolu wrote:
> When the first level page table is used for IOVA translation, it only
> supports Read-Only and Read-Write permissions. The Write-Only permission
> is not supported as the PRESENT bit (implying Read permission) should
> always set. When using second level, we still give separate permissions
> that allows WriteOnly which seems inconsistent and awkward. There is no
> use case we can think off, hence remove that configuration to make it
> consistent.

No use-case for WriteOnly mappings? How about DMA_FROM_DEVICE mappings?