[kvm-unit-tests PATCH v1 18/18] arm/arm64: Rework the cache maintenance in asm_mmu_disable

Shaoqin Huang
2023-11-30T09:08:01Z

asm_mmu_disable is overly ambitious and provably incorrect:

1. It tries to clean and invalidate the data caches for the **entire**
memory, which is highly unnecessary, as it's very unlikely that a test
will write to the entire memory, and even more unlikely that a test will
modify the text section of the test image.

2. There is no corresponding dcache invalidate command for the entire
memory in asm_mmu_enable, leaving it up to the test that disabled the
MMU to do the cache maintenance in an asymmetrical fashion: only for
re-enabling the MMU, but not for disabling it.

3. It's missing the DMB SY memory barrier to ensure that the dcache
maintenance is performed after the last store executed in program order
before calling asm_mmu_disable.

Fix all of the issues in one go, by doing the cache maintenance only for
the stack, as that is out of the control of the C code, and add the missing
memory barrier. A test that disables the MMU is now expected to do whatever
cache maintenance is necessary to execute correctly after the MMU is
disabled.

The code used to test that mmu_disable works correctly is similar to the
code used to test commit 410b3bf09e76 ("arm/arm64: Perform dcache clean
+ invalidate after turning MMU off"), with extra cache maintenance
added:

+#include <alloc_page.h>
+#include <asm/cacheflush.h>
+#include <asm/mmu.h>
int main(int argc, char **argv)
{
+ int *x = alloc_page();
+ bool pass = true;
+ int i;
+
+ for (i = 0; i < 1000000; i++) {
+ *x = 0x42;
+ dcache_clean_addr_poc((unsigned long)x);
+ mmu_disable();
+ if (*x != 0x42) {
+ mmu_enable(current_thread_info()->pgtable);
+ pass = false;
+ break;
+ }
+ *x = 0x50;
+ /* Needed for the invalidation only. */
+ dcache_clean_inval_addr_poc((unsigned long)x);
+ mmu_enable(current_thread_info()->pgtable);
+ if (*x != 0x50) {
+ pass = false;
+ break;
+ }
+ }
+ report(pass, "MMU disable cache maintenance");

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
---
arm/cstart.S | 19 +++++++++++++------
arm/cstart64.S | 19 ++++++++++++-------
2 files changed, 25 insertions(+), 13 deletions(-)

diff --git a/arm/cstart.S b/arm/cstart.S
index 48dc87f5..3950e45e 100644
--- a/arm/cstart.S
+++ b/arm/cstart.S
@@ -240,18 +240,25 @@ asm_mmu_enable:

.globl asm_mmu_disable
asm_mmu_disable:
+ /*
+ * A test can change the memory attributes for a memory location to
+ * Device or Inner Non-cacheable, which makes the barrier required to
+ * avoid reordering of previous memory accesses with respect to the
+ * cache maintenance.
+ */
+ dmb sy
+ mov r0, sp
+ lsr r0, #THREAD_SHIFT
+ lsl r0, #THREAD_SHIFT
+ add r1, r0, #THREAD_SIZE
+ dcache_by_line_op dccmvac, sy, r0, r1, r2, r3
+
/* SCTLR */
mrc p15, 0, r0, c1, c0, 0
bic r0, #CR_M
mcr p15, 0, r0, c1, c0, 0
isb

- ldr r0, =__phys_offset
- ldr r0, [r0]
- ldr r1, =__phys_end
- ldr r1, [r1]
- dcache_by_line_op dccimvac, sy, r0, r1, r2, r3
-
mov pc, lr

/*
diff --git a/arm/cstart64.S b/arm/cstart64.S
index d8200ea2..af56186c 100644
--- a/arm/cstart64.S
+++ b/arm/cstart64.S
@@ -288,18 +288,23 @@ asm_mmu_enable:

.globl asm_mmu_disable
asm_mmu_disable:
+ /*
+ * A test can change the memory attributes for a memory location to
+ * Device or Inner Non-cacheable, which makes the barrier required to
+ * avoid reordering of previous memory accesses with respect to the
+ * cache maintenance.
+ */
+ dmb sy
+ mov x9, sp
+ and x9, x9, #THREAD_MASK
+ add x10, x9, #THREAD_SIZE
+ dcache_by_line_op cvac, sy, x9, x10, x11, x12
+
mrs x0, sctlr_el1
bic x0, x0, SCTLR_EL1_M
msr sctlr_el1, x0
isb

- /* Clean + invalidate the entire memory */
- adrp x0, __phys_offset
- ldr x0, [x0, :lo12:__phys_offset]
- adrp x1, __phys_end
- ldr x1, [x1, :lo12:__phys_end]
- dcache_by_line_op civac, sy, x0, x1, x2, x3
-
ret

/*
--
2.40.1