linux-kernel - Re: [PATCH 4/4] thunderbolt: Export IOMMU based DMA protection support to userspace

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Message-ID: <CA+CmpXtre+A-sNT0XBWfdkuq9xJ+j34EGZ0nEpahJCSe3Kvg9A@mail.gmail.com>
Date:   Tue, 13 Nov 2018 13:13:31 +0200
From:   Yehezkel Bernat <yehezkelshb@...il.com>
To:     Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:     iommu@...ts.linux-foundation.org, joro@...tes.org,
        David Woodhouse <dwmw2@...radead.org>,
        baolu.lu@...ux.intel.com, ashok.raj@...el.com,
        Bjorn Helgaas <bhelgaas@...gle.com>, rjw@...ysocki.net,
        jacob.jun.pan@...el.com, Andreas Noever <andreas.noever@...il.com>,
        michael.jamet@...el.com, lukas@...ner.de,
        Christian Kellner <ckellner@...hat.com>,
        Mario Limonciello <Mario.Limonciello@...l.com>,
        Anthony Wong <anthony.wong@...onical.com>,
        linux-acpi@...r.kernel.org, linux-pci@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] thunderbolt: Export IOMMU based DMA protection
 support to userspace

On Tue, Nov 13, 2018 at 12:56 PM Mika Westerberg
<mika.westerberg@...ux.intel.com> wrote:
>
> > Just one point:
> > Have you considered the option to add this property per (TBT?) device?
>
> No. ;-)
>
> You mean that one device uses security levels and another IOMMU? I don't
> think it is possible without having some sort of table in the IOMMU
> driver telling which devices it needs identity map and which not. Also
> not sure what would be the benefit?

For performance, of course. If some devices are considered safe (maybe a list
communicated by platform firmware), the kernel may decide to configure them to
passthrough the IOMMU (I think I remember there is such an option, but maybe I'm
wrong.)


> > If the kernel may decide to enable/disable the IOMMU or AST per device, maybe
> > it should be on this level.
> > Or maybe the IOMMU decision isn't going to change (it's system-wide) and the AST
> > decision will be communicated per device by a new sysfs attribute anyway, if
> > needed?
>
> Not sure what you mean by "AST"? The IOMMU decision is pretty much
> system-wide.

Sorry, I meant ATS, Address Translation Service, mentioned in patch 3 in this
series, and possibly be enabled for some devices for performance, as mentioned
there.
So if needed, this will be another attribute, and definitely
per-device, isn't it?