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

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Message-ID: <CA+CmpXt1cfqjz94=WsggKpNCUw3761-cewa_HTa0ETcYK7o+Hw@mail.gmail.com>
Date:   Tue, 13 Nov 2018 16:42:58 +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 1:40 PM Mika Westerberg
<mika.westerberg@...ux.intel.com> wrote:
>
> On Tue, Nov 13, 2018 at 01:13:31PM +0200, Yehezkel Bernat wrote:
> > 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.)
>
> At least I'm not aware of such an option. Windows for example enables
> IOMMU for everything and I think macOS does the same. In Linux (with
> these patches) we put all internal devices already passthrough mode so
> things like internal graphics should not be affected. eGPUs are
> different thing, though.

So your point here is "currently we do the IOMMU decisions system-wide; we can
always add a per-device attribute if needed"?
Fair enough.

So for this patch,
Reviewed-by: Yehezkel Bernat <YehezkelShB@...il.com>