CONFIG_ARM_SMMU_DISABLE_BYPASS_BY_DEFAULT: Disable unmatched stream bypass by default

General informations

The Linux kernel configuration item CONFIG_ARM_SMMU_DISABLE_BYPASS_BY_DEFAULT has multiple definitions:

Disable unmatched stream bypass by default found in drivers/iommu/arm/Kconfig

The configuration item CONFIG_ARM_SMMU_DISABLE_BYPASS_BY_DEFAULT:

• prompt: Disable unmatched stream bypass by default
• type: bool
• depends on: CONFIG_EXPERT
• defined in drivers/iommu/arm/Kconfig
• found in Linux kernels: 6.16-rc+HEAD

Help text

If your firmware is broken and fails to describe StreamIDs which Linux should know about in order to manage the SMMU correctly and securely, and you don't want to boot with the 'arm-smmu.disable_bypass=0' command line parameter, then as a last resort you can turn it off by default here. But don't. This option may be removed at any time.

Note that 'arm-smmu.disable_bypass=1' will still take precedence.

Default to disabling bypass on ARM SMMU v1 and v2 found in drivers/iommu/Kconfig

The configuration item CONFIG_ARM_SMMU_DISABLE_BYPASS_BY_DEFAULT:

• prompt: Default to disabling bypass on ARM SMMU v1 and v2
• type: bool
• depends on: CONFIG_ARM_SMMU
• defined in drivers/iommu/Kconfig
• found in Linux kernels: 5.2–5.19, 6.0–6.15

Help text

Say Y here to (by default) disable bypass streams such that incoming transactions from devices that are not attached to an iommu domain will report an abort back to the device and will not be allowed to pass through the SMMU.

Any old kernels that existed before this KConfig was introduced would default to _allowing_ bypass (AKA the equivalent of NO for this config). However the default for this option is YES because the old behavior is insecure.

There are few reasons to allow unmatched stream bypass, and even fewer good ones. If saying YES here breaks your board you should work on fixing your board. This KConfig option is expected to be removed in the future and we'll simply hardcode the bypass disable in the code.

NOTE: the kernel command line parameter 'arm-smmu.disable_bypass' will continue to override this config.

Hardware

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The data is retrived from:

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