arch: arm: mpu: introduce option to skip background MPU region fill
We introduce MPU_GAP_FILLING Kconfig option that instructs the MPU driver to enforce a full SRAM partitioning, when it programs the dynamic MPU regions (user thread stack, PRIV stack guard and application memory domains) at context-switch. We allow this to be configurable, in order to increase the number of MPU regions available for application memory domain programming. This option is introduced in arch/Kconfig, as it is expected to serve as a cross-ARCH symbol. The option can be set by the user during build configuration. By not enforcing full partition, we may leave part of kernel SRAM area covered only by the default ARM memory map. This is fine for User Mode, since the background ARM map does not allow nPRIV access at all. The difference is that kernel code will be able to attempt fetching instructions from kernel SRAM area without this leading directly to a MemManage exception. Since this does not compromize User Mode, we make the skipping of full partitioning the default behavior for the ARMv8-M MPU driver. The application developer may be able to overwrite this. In the wake of this change we update the macro definitions in arm_core_mpu_dev.h that derive the maximum number of MPU regions for application memory domains. Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>
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3 changed files with 55 additions and 6 deletions
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arch/Kconfig
23
arch/Kconfig
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@ -449,6 +449,29 @@ config MPU_REQUIRES_NON_OVERLAPPING_REGIONS
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This option is enabled when the MPU requires the active (i.e. enabled)
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MPU regions to be non-overlapping with each other.
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config MPU_GAP_FILLING
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bool "Force MPU to be filling in background memory regions"
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depends on MPU_REQUIRES_NON_OVERLAPPING_REGIONS
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depends on USERSPACE
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help
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This Kconfig option instructs the MPU driver to enforce
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a full kernel SRAM partitioning, when it programs the
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dynamic MPU regions (user thread stack, PRIV stack guard
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and application memory domains) during context-switch. We
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allow this to be a configurable option, in order to be able
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to switch the option off and have an increased number of MPU
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regions available for application memory domain programming.
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Notes:
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An increased number of MPU regions should only be required,
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when building with USERSPACE support.
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When the option is switched off, access to memory areas not
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covered by explicit MPU regions is restricted to privileged
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code on an ARCH-specific basis. Refer to ARCH-specific
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documentation for more information on how this option is
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used.
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menuconfig FLOAT
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bool "Floating point"
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depends on CPU_HAS_FPU
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@ -14,10 +14,33 @@ config ARM_MPU
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select MPU_REQUIRES_NON_OVERLAPPING_REGIONS if CPU_HAS_ARM_MPU && (ARMV8_M_BASELINE || ARMV8_M_MAINLINE)
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help
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MCU implements Memory Protection Unit.
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Note that NXP MPU as well as ARMv8-M MPU does not require MPU regions
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Notes:
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The ARMv6-M and ARMv8-M MPU architecture requires a power-of-two
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alignment of MPU region base address and size.
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The NXP MPU as well as the ARMv8-M MPU do not require MPU regions
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to have power-of-two alignment for base address and region size.
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In addition to the above, ARMv8-M MPU requires the active MPU regions
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be non-overlapping.
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The ARMv8-M MPU requires the active MPU regions be non-overlapping.
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As a result of this, the ARMv8-M MPU needs to fully partition the
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memory map when programming dynamic memory regions (e.g. PRIV stack
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guard, user thread stack, and application memory domains), if the
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system requires PRIV access policy different from the access policy
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of the ARMv8-M background memory map. The allication developer may
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enforce full PRIV (kernel) memory partition by enabling the
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CONFIG_MPU_GAP_FILLING option.
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By not enforcing full partition, MPU may leave part of kernel
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SRAM area covered only by the default ARMv8-M memory map. This
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is fine for User Mode, since the background ARM map does not
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allow nPRIV access at all. However, since the background map
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policy allows instruction fetches by privileged code, forcing
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this Kconfig option off prevents the system from directly
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triggering MemManage exceptions upon accidental attempts to
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execute code from SRAM in XIP builds.
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Since this does not compromise User Mode, we make the skipping
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of full partitioning the default behavior for the ARMv8-M MPU
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driver.
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config ARM_MPU_REGION_MIN_ALIGN_AND_SIZE
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int
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@ -28,7 +28,8 @@ struct k_thread;
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*
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* @param mpu_regions_num the number of available HW MPU regions.
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*/
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#if defined(CONFIG_MPU_REQUIRES_NON_OVERLAPPING_REGIONS)
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#if defined(CONFIG_MPU_REQUIRES_NON_OVERLAPPING_REGIONS) && \
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!defined(CONFIG_MPU_SKIP_BACKGROUND_SRAM_FILLING)
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/*
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* For ARM MPU architectures, where the domain partitions cannot be defined
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* on top of the statically configured memory regions, the maximum number of
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@ -53,7 +54,8 @@ struct k_thread;
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* @brief Maximum number of MPU regions required to configure a
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* memory region for (user) Thread Stack.
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*/
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#if defined(CONFIG_MPU_REQUIRES_NON_OVERLAPPING_REGIONS)
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#if defined(CONFIG_MPU_REQUIRES_NON_OVERLAPPING_REGIONS) && \
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!defined(CONFIG_MPU_SKIP_BACKGROUND_SRAM_FILLING)
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/* When dynamic regions may not be defined on top of statically
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* allocated memory regions, defining a region for a thread stack
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* requires two additional MPU regions to be configured; one for
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@ -73,7 +75,8 @@ struct k_thread;
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* @brief Maximum number of MPU regions required to configure a
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* memory region for a (supervisor) Thread Stack Guard.
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*/
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#if defined(CONFIG_MPU_REQUIRES_NON_OVERLAPPING_REGIONS) \
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#if (defined(CONFIG_MPU_REQUIRES_NON_OVERLAPPING_REGIONS) && \
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!defined(CONFIG_MPU_SKIP_BACKGROUND_SRAM_FILLING)) \
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|| defined(CONFIG_CPU_HAS_NXP_MPU)
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/*
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* When dynamic regions may not be defined on top of statically
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