linux-kernel - Re: [PATCH v4 4/4] PCI/ACS: Enable PCI_ACS_TB for untrusted/external-facing devices

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Date:   Mon, 13 Jul 2020 10:01:15 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Rajat Jain <rajatxjain@...il.com>, Rajat Jain <rajatja@...gle.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        "open list:AMD IOMMU (AMD-VI)" <iommu@...ts.linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        "Krishnakumar, Lalithambika" <lalithambika.krishnakumar@...el.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Prashant Malani <pmalani@...gle.com>,
        Benson Leung <bleung@...gle.com>,
        Todd Broch <tbroch@...gle.com>,
        Alex Levin <levinale@...gle.com>,
        Mattias Nissler <mnissler@...gle.com>,
        Bernie Keany <bernie.keany@...el.com>,
        Aaron Durbin <adurbin@...gle.com>,
        Diego Rivas <diegorivas@...gle.com>,
        Duncan Laurie <dlaurie@...gle.com>,
        Furquan Shaikh <furquan@...gle.com>,
        Jesse Barnes <jsbarnes@...gle.com>,
        Christian Kellner <christian@...lner.me>,
        Alex Williamson <alex.williamson@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Oliver O'Halloran <oohall@...il.com>,
        Saravana Kannan <saravanak@...gle.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Subject: Re: [PATCH v4 4/4] PCI/ACS: Enable PCI_ACS_TB for
 untrusted/external-facing devices

On Sat, Jul 11, 2020 at 09:58:38PM -0500, Bjorn Helgaas wrote:
> If BIOS handed off with ATS enabled and we somehow relied on it being
> already enabled, something might break if we start disabling ATS.
> Just a theoretical possibility, doesn't seem likely to me.

I don't think this will be a problem. When the BIOS enables ATS for a
device it also needs to enable the IOMMU already, an we are not handling
an already enabled IOMMU (outside of kdump kernels) very well.


Regards,

	Joerg