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

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Message-ID: <20200710202922.GA77140@bjorn-Precision-5520>
Date:   Fri, 10 Jul 2020 15:29:22 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Rajat Jain <rajatja@...gle.com>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-acpi@...r.kernel.org, Raj Ashok <ashok.raj@...el.com>,
        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>,
        Rajat Jain <rajatxjain@...il.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>,
        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 Tue, Jul 07, 2020 at 03:46:04PM -0700, Rajat Jain wrote:
> When enabling ACS, enable translation blocking for external facing ports
> and untrusted devices.
> 
> Signed-off-by: Rajat Jain <rajatja@...gle.com>
> ---
> v4: Add braces to avoid warning from kernel robot
>     print warning for only external-facing devices.
> v3: print warning if ACS_TB not supported on external-facing/untrusted ports.
>     Minor code comments fixes.
> v2: Commit log change
> 
>  drivers/pci/pci.c    |  8 ++++++++
>  drivers/pci/quirks.c | 15 +++++++++++++++
>  2 files changed, 23 insertions(+)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 73a8627822140..a5a6bea7af7ce 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -876,6 +876,14 @@ static void pci_std_enable_acs(struct pci_dev *dev)
>  	/* Upstream Forwarding */
>  	ctrl |= (cap & PCI_ACS_UF);
>  
> +	/* Enable Translation Blocking for external devices */
> +	if (dev->external_facing || dev->untrusted) {
> +		if (cap & PCI_ACS_TB)
> +			ctrl |= PCI_ACS_TB;
> +		else if (dev->external_facing)
> +			pci_warn(dev, "ACS: No Translation Blocking on external-facing dev\n");
> +	}

IIUC, this means that external devices can *never* use ATS and can
never cache translations.  And (I guess, I'm not an expert) it can
also never use the Page Request Services?

Is this what we want?  Do we have any idea how many external devices
this will affect or how much of a performance impact they will see?

Do we need some kind of override or mechanism to authenticate certain
devices so they can use ATS and PRI?

If we do decide this is the right thing to do, I think we need to
expand the commit log a bit, because this is potentially a significant
user-visible change.

>  	pci_write_config_word(dev, pos + PCI_ACS_CTRL, ctrl);
>  }
>  
> diff --git a/drivers/pci/quirks.c b/drivers/pci/quirks.c
> index b341628e47527..bb22b46c1d719 100644
> --- a/drivers/pci/quirks.c
> +++ b/drivers/pci/quirks.c
> @@ -4934,6 +4934,13 @@ static void pci_quirk_enable_intel_rp_mpc_acs(struct pci_dev *dev)
>  	}
>  }
>  
> +/*
> + * Currently this quirk does the equivalent of
> + * PCI_ACS_SV | PCI_ACS_RR | PCI_ACS_CR | PCI_ACS_UF
> + *
> + * TODO: This quirk also needs to do equivalent of PCI_ACS_TB,
> + * if dev->external_facing || dev->untrusted
> + */
>  static int pci_quirk_enable_intel_pch_acs(struct pci_dev *dev)
>  {
>  	if (!pci_quirk_intel_pch_acs_match(dev))
> @@ -4973,6 +4980,14 @@ static int pci_quirk_enable_intel_spt_pch_acs(struct pci_dev *dev)
>  	ctrl |= (cap & PCI_ACS_CR);
>  	ctrl |= (cap & PCI_ACS_UF);
>  
> +	/* Enable Translation Blocking for external devices */
> +	if (dev->external_facing || dev->untrusted) {
> +		if (cap & PCI_ACS_TB)
> +			ctrl |= PCI_ACS_TB;
> +		else if (dev->external_facing)
> +			pci_warn(dev, "ACS: No Translation Blocking on external-facing dev\n");
> +	}
> +
>  	pci_write_config_dword(dev, pos + INTEL_SPT_ACS_CTRL, ctrl);
>  
>  	pci_info(dev, "Intel SPT PCH root port ACS workaround enabled\n");
> -- 
> 2.27.0.212.ge8ba1cc988-goog
>