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Re: [PATCH v3 2/7] arm/mpu: Provide access to the MPU region from the C code
Hi Luca,
On 11/04/2025 23:56, Luca Fancellu wrote:
Implement some utility function in order to access the MPU regions
from the C world.
Signed-off-by: Luca Fancellu <luca.fancellu@xxxxxxx>
---
v3 changes:
- Moved PRBAR0_EL2/PRLAR0_EL2 to arm64 specific
- Modified prepare_selector() to be easily made a NOP
for Arm32, which can address up to 32 region without
changing selector and it is also its maximum amount
of MPU regions.
---
---
xen/arch/arm/include/asm/arm64/mpu.h | 7 ++
xen/arch/arm/include/asm/mpu.h | 1 +
xen/arch/arm/include/asm/mpu/mm.h | 24 +++++
xen/arch/arm/mpu/mm.c | 125 +++++++++++++++++++++++++++
4 files changed, 157 insertions(+)
diff --git a/xen/arch/arm/include/asm/arm64/mpu.h
b/xen/arch/arm/include/asm/arm64/mpu.h
index 4d2bd7d7877f..b4e1ecdf741d 100644
--- a/xen/arch/arm/include/asm/arm64/mpu.h
+++ b/xen/arch/arm/include/asm/arm64/mpu.h
@@ -8,6 +8,13 @@
#ifndef __ASSEMBLY__
+/*
+ * The following are needed for the case generators GENERATE_WRITE_PR_REG_CASE
+ * and GENERATE_READ_PR_REG_CASE with num==0
+ */
+#define PRBAR0_EL2 PRBAR_EL2
+#define PRLAR0_EL2 PRLAR_EL2
Rather than aliasing, shouldn't we just rename PR{B,L}AR_EL2 to
PR{B,L}AR0_EL2? This would the code mixing between the two.
+
/* Protection Region Base Address Register */
typedef union {
struct __packed {
diff --git a/xen/arch/arm/include/asm/mpu.h b/xen/arch/arm/include/asm/mpu.h
index e148c705b82c..59ff22c804c1 100644
--- a/xen/arch/arm/include/asm/mpu.h
+++ b/xen/arch/arm/include/asm/mpu.h
@@ -13,6 +13,7 @@
#define MPU_REGION_SHIFT 6
#define MPU_REGION_ALIGN (_AC(1, UL) << MPU_REGION_SHIFT)
#define MPU_REGION_MASK (~(MPU_REGION_ALIGN - 1))
+#define MPU_REGION_RES0 (0xFFFULL << 52)
#define NUM_MPU_REGIONS_SHIFT 8
#define NUM_MPU_REGIONS (_AC(1, UL) << NUM_MPU_REGIONS_SHIFT)
diff --git a/xen/arch/arm/include/asm/mpu/mm.h
b/xen/arch/arm/include/asm/mpu/mm.h
index 86f33d9836b7..5cabe9d111ce 100644
--- a/xen/arch/arm/include/asm/mpu/mm.h
+++ b/xen/arch/arm/include/asm/mpu/mm.h
@@ -8,6 +8,7 @@
#include <xen/page-size.h>
#include <xen/types.h>
#include <asm/mm.h>
+#include <asm/mpu.h>
extern struct page_info *frame_table;
@@ -29,6 +30,29 @@ static inline struct page_info *virt_to_page(const void *v)
return mfn_to_page(mfn);
}
+/* Utility function to be used whenever MPU regions are modified */
+static inline void context_sync_mpu(void)
+{
+ /*
+ * ARM DDI 0600B.a, C1.7.1
+ * Writes to MPU registers are only guaranteed to be visible following a
+ * Context synchronization event and DSB operation.
I know we discussed about this before. I find odd that the specification
says "context synchronization event and DSB operation". At least to me,
it implies "isb + dsb" not the other way around. Has this been clarified
in newer version of the specification?
+ */
+ dsb(sy);
+ isb();
+}
+
+/*
+ * The following API require context_sync_mpu() after being used to modifiy MPU
typo: s/require/requires/ and s/modifiy/modify/
+ * regions:
+ * - write_protection_region
+ */
+
+/* Reads the MPU region with index 'sel' from the HW */
+extern void read_protection_region(pr_t *pr_read, uint8_t sel);
I am probably missing something. But don't you have a copy of pr_t in
xen_mpumap? If so, can't we use the cached version to avoid accessing
the system registers?
+/* Writes the MPU region with index 'sel' to the HW */
+extern void write_protection_region(const pr_t *pr_write, uint8_t sel);
+
#endif /* __ARM_MPU_MM_H__ */
/*
diff --git a/xen/arch/arm/mpu/mm.c b/xen/arch/arm/mpu/mm.c
index f83ce04fef8a..e522ce53c357 100644
--- a/xen/arch/arm/mpu/mm.c
+++ b/xen/arch/arm/mpu/mm.c
@@ -8,12 +8,30 @@
#include <xen/sizes.h>
#include <xen/types.h>
#include <asm/mpu.h>
+#include <asm/mpu/mm.h>
+#include <asm/sysregs.h>
struct page_info *frame_table;
/* EL2 Xen MPU memory region mapping table. */
pr_t xen_mpumap[MAX_MPU_REGIONS];
+#define GENERATE_WRITE_PR_REG_CASE(num, pr) \
+ case num: \
+ { \
+ WRITE_SYSREG(pr->prbar.bits & ~MPU_REGION_RES0, PRBAR##num##_EL2); \
+ WRITE_SYSREG(pr->prlar.bits & ~MPU_REGION_RES0, PRLAR##num##_EL2); \
+ break; \
+ }
+
+#define GENERATE_READ_PR_REG_CASE(num, pr) \
+ case num: \
+ { \
+ pr->prbar.bits = READ_SYSREG(PRBAR##num##_EL2); \
+ pr->prlar.bits = READ_SYSREG(PRLAR##num##_EL2); \
+ break; \
+ }
+
static void __init __maybe_unused build_assertions(void)
{
/*
@@ -24,6 +42,113 @@ static void __init __maybe_unused build_assertions(void)
BUILD_BUG_ON(PAGE_SIZE != SZ_4K);
}
+static void prepare_selector(uint8_t *sel)
+{
+ uint8_t cur_sel = *sel;
Coding style: Missing newline.
+ /*
+ * {read,write}_protection_region works using the direct access to the
0..15
+ * regions, so in order to save the isb() overhead, change the PRSELR_EL2
+ * only when needed, so when the upper 4 bits of the selector will change.
+ */
+ cur_sel &= 0xF0U;
+ if ( READ_SYSREG(PRSELR_EL2) != cur_sel )
+ {
+ WRITE_SYSREG(cur_sel, PRSELR_EL2);
+ isb();
+ }
+ *sel = *sel & 0xFU;
+}
+
+/*
+ * Armv8-R AArch64 at most supports 255 MPU protection regions.
+ * See section G1.3.18 of the reference manual for Armv8-R AArch64,
+ * PRBAR<n>_EL2 and PRLAR<n>_EL2 provide access to the EL2 MPU region
+ * determined by the value of 'n' and PRSELR_EL2.REGION as
+ * PRSELR_EL2.REGION<7:4>:n(n = 0, 1, 2, ... , 15)
+ * For example to access regions from 16 to 31 (0b10000 to 0b11111):
+ * - Set PRSELR_EL2 to 0b1xxxx
+ * - Region 16 configuration is accessible through PRBAR_EL2 and PRLAR_EL2
+ * - Region 17 configuration is accessible through PRBAR1_EL2 and PRLAR1_EL2
+ * - Region 18 configuration is accessible through PRBAR2_EL2 and PRLAR2_EL2
+ * - ...
+ * - Region 31 configuration is accessible through PRBAR15_EL2 and PRLAR15_EL2
+ */
I am a bit confused. This function is implemented in the common MPU
code. Yet, then comment only refer to 64-bit. Is the code the same on
32-bit? If not, then I think this function wants to be moved in arm64/mpu/
+/*
+ * Read EL2 MPU Protection Region.
+ *
+ * @pr_read: mpu protection region returned by read op.
+ * @sel: mpu protection region selector
+ */
NIT: Usually we add documentation on the prototype in the header and not
in the definition.
+void read_protection_region(pr_t *pr_read, uint8_t sel)
+{
+ /*
+ * Before accessing EL2 MPU region register PRBAR_EL2/PRLAR_EL2,
+ * make sure PRSELR_EL2 is set, as it determines which MPU region
+ * is selected.
+ */
+ prepare_selector(&sel);
+
+ switch ( sel )
+ {
+ GENERATE_READ_PR_REG_CASE(0, pr_read);
+ GENERATE_READ_PR_REG_CASE(1, pr_read);
+ GENERATE_READ_PR_REG_CASE(2, pr_read);
+ GENERATE_READ_PR_REG_CASE(3, pr_read);
+ GENERATE_READ_PR_REG_CASE(4, pr_read);
+ GENERATE_READ_PR_REG_CASE(5, pr_read);
+ GENERATE_READ_PR_REG_CASE(6, pr_read);
+ GENERATE_READ_PR_REG_CASE(7, pr_read);
+ GENERATE_READ_PR_REG_CASE(8, pr_read);
+ GENERATE_READ_PR_REG_CASE(9, pr_read);
+ GENERATE_READ_PR_REG_CASE(10, pr_read);
+ GENERATE_READ_PR_REG_CASE(11, pr_read);
+ GENERATE_READ_PR_REG_CASE(12, pr_read);
+ GENERATE_READ_PR_REG_CASE(13, pr_read);
+ GENERATE_READ_PR_REG_CASE(14, pr_read);
+ GENERATE_READ_PR_REG_CASE(15, pr_read);
+ default:
+ BUG(); /* Can't happen */
+ }
+}
+
+/*
+ * Write EL2 MPU Protection Region.
+ *
+ * @pr_write: const mpu protection region passed through write op.
+ * @sel: mpu protection region selector
+ */
+void write_protection_region(const pr_t *pr_write, uint8_t sel)
+{
+ /*
+ * Before accessing EL2 MPU region register PRBAR_EL2/PRLAR_EL2,
+ * make sure PRSELR_EL2 is set, as it determines which MPU region
+ * is selected.
+ */
+ prepare_selector(&sel);
+
+ switch ( sel )
+ {
+ GENERATE_WRITE_PR_REG_CASE(0, pr_write);
+ GENERATE_WRITE_PR_REG_CASE(1, pr_write);
+ GENERATE_WRITE_PR_REG_CASE(2, pr_write);
+ GENERATE_WRITE_PR_REG_CASE(3, pr_write);
+ GENERATE_WRITE_PR_REG_CASE(4, pr_write);
+ GENERATE_WRITE_PR_REG_CASE(5, pr_write);
+ GENERATE_WRITE_PR_REG_CASE(6, pr_write);
+ GENERATE_WRITE_PR_REG_CASE(7, pr_write);
+ GENERATE_WRITE_PR_REG_CASE(8, pr_write);
+ GENERATE_WRITE_PR_REG_CASE(9, pr_write);
+ GENERATE_WRITE_PR_REG_CASE(10, pr_write);
+ GENERATE_WRITE_PR_REG_CASE(11, pr_write);
+ GENERATE_WRITE_PR_REG_CASE(12, pr_write);
+ GENERATE_WRITE_PR_REG_CASE(13, pr_write);
+ GENERATE_WRITE_PR_REG_CASE(14, pr_write);
+ GENERATE_WRITE_PR_REG_CASE(15, pr_write);
+ default:
+ BUG(); /* Can't happen */
+ }
+}
+
void __init setup_mm(void)
{
BUG_ON("unimplemented");
Cheers,
--
Julien Grall
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