linux-kernel - Re: [PATCH v2] iommu/amd: Set translation valid bit only when IO page tables are in used

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Message-ID: <Yn5YIfzmerwAM30M@8bytes.org>
Date:   Fri, 13 May 2022 15:07:45 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Suravee Suthikulpanit <suravee.suthikulpanit@....com>
Cc:     linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        vasant.hegde@....com, jon.grimm@....com
Subject: Re: [PATCH v2] iommu/amd: Set translation valid bit only when IO
 page tables are in used

On Mon, May 09, 2022 at 02:48:15AM -0500, Suravee Suthikulpanit wrote:
> On AMD system with SNP enabled, IOMMU hardware checks the host translation
> valid (TV) and guest translation valid (GV) bits in the device
> table entry (DTE) before accessing the corresponded page tables.
> 
> However, current IOMMU driver sets the TV bit for all devices
> regardless of whether the host page table is in used.
> This results in ILLEGAL_DEV_TABLE_ENTRY event for devices, which
> do not the host page table root pointer set up.

Hmm, this sound weird. In the early AMD IOMMUs it was recommended to set
TV=1 and V=1 and the rest to 0 to block all DMA from a device.

I wonder how this triggers ILLEGAL_DEV_TABLE_ENTRY errors now. It is
(was?) legal to set V=1 TV=1, mode=0 and leave the page-table empty.
When then IW=0 and IR=0, DMA is blocked. From what I remember this is a
valid setting in a DTE. Do you have an example DTE which triggers this
error message?

Regards,

	Joerg