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aarch64: Fix alignment fault on z_bss_zero()

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Using newlibc with AArch64 is causing an alignement fault in
z_bss_zero() when the code is run on real hardware (on QEMU the problem
is not reproducible).

The main cause is that the memset() function exported by newlibc is
using 'DC ZVA' to zero out memory.

While this is often a nice optimization, this is causing the issue on
AArch64 because memset() is being used before the MMU is enabled, and
when the MMU is disabled all data accesses will be treated as
Device_nGnRnE.

This is a problem because quoting from the ARM reference manual: "If the
memory region being zeroed is any type of Device memory, then DC ZVA
generates an Alignment fault which is prioritized in the same way as
other alignment faults that are determined by the memory type".

newlibc tries to be a bit smart about this reading the DCZID_EL0
register before deciding whether using 'DC ZVA' or not. While this is a
good idea for code running in EL0, currently the Zephyr kernel is
running in EL1. This means that the value of the DCZID_EL0 register is
actually retrieved from the HCR_EL2.TDZ bit, that is always 0 because
EL2 is not currently supported / enabled. So the 'DC ZVA' instruction is
unconditionally used in the newlibc memset() implementation.

The "standard" solution for this case is usually to use a different
memset routine to be specifically used for two cases: (1) against IO
memory or (2) against normal memory but with MMU disabled (which means
all memory is considered device memory for data accesses).

To fix this issue in Zephyr we avoid calling memset() when clearing the
bss, and instead we use a simple loop to zero the memory region.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>
This commit is contained in:
Carlo Caione 2021-01-14 21:28:55 +01:00 • committed by Andrew Boie
commit c5b898743a
1 changed files with 13 additions and 1 deletions
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14
arch/arm/core/aarch64/prep_c.c
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@ -15,8 +15,20 @@
*/
#include <kernel_internal.h>
#include <linker/linker-defs.h>
extern FUNC_NORETURN void z_cstart(void);
static inline void z_arm64_bss_zero(void)
{
uint64_t *p = (uint64_t *)__bss_start;
uint64_t *end = (uint64_t *)__bss_end;
while (p < end) {
*p++ = 0;
}
}
/**
*
* @brief Prepare to and run C code
@ -27,7 +39,7 @@ extern FUNC_NORETURN void z_cstart(void);
*/
void z_arm64_prep_c(void)
{
z_bss_zero();
z_arm64_bss_zero();
z_arm64_interrupt_init();
z_cstart();