On Tue, Aug 09, 2022 at 10:15:55AM +0100, Alexandru Elisei wrote:
> The vmalloc allocator returns non-id mapped addresses, where the virtual
> address is different than the physical address. This makes it impossible
> to access the stack of the secondary CPUs while the MMU is disabled.
>
> On arm, THREAD_SIZE is 16K and PAGE_SIZE is 4K, which makes THREAD_SIZE
> a power of two multiple of PAGE_SIZE. On arm64, THREAD_SIZE is 16 when
> PAGE_SIZE is 4K or 16K, and 64K when PAGE_SIZE is 64K. In all cases,
> THREAD_SIZE is a power of two multiple of PAGE_SIZE. As a result, using
> memalign_pages() for the stack won't lead to wasted memory.
>
> memalign_pages() allocates memory in chunks of power of two number of
> pages, aligned to the allocation size, which makes it a drop-in
> replacement for vm_memalign (which is the value for alloc_ops->memalign
> when the stack is allocated).
>
> Using memalign_pages() has two distinct benefits:
>
> 1. The secondary CPUs' stack can be used with the MMU off.
>
> 2. The secondary CPUs' stack is identify mapped similar to the stack for
> the primary CPU, which makes the configuration of the CPUs consistent.
>
> memalign_pages_flags() has been used instead of memalign_pages() to
> instruct the allocator not to zero the stack, as it's already zeroed in the
> entry code.
>
> Signed-off-by: Alexandru Elisei <alexandru.elisei@xxxxxxx>
> ---
> lib/arm/asm/thread_info.h | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/lib/arm/asm/thread_info.h b/lib/arm/asm/thread_info.h
> index eaa72582af86..190e082cbba0 100644
> --- a/lib/arm/asm/thread_info.h
> +++ b/lib/arm/asm/thread_info.h
> @@ -25,6 +25,7 @@
> #ifndef __ASSEMBLY__
> #include <asm/processor.h>
> #include <alloc.h>
> +#include <alloc_page.h>
>
> #ifdef __arm__
> #include <asm/ptrace.h>
> @@ -40,7 +41,7 @@
>
> static inline void *thread_stack_alloc(void)
> {
> - void *sp = memalign(THREAD_ALIGNMENT, THREAD_SIZE);
> + void *sp = memalign_pages_flags(THREAD_ALIGNMENT, THREAD_SIZE, FLAG_DONTZERO);
> return sp + THREAD_START_SP;
> }
>
> --
> 2.37.1
>
Reviewed-by: Andrew Jones <andrew.jones@xxxxxxxxx>
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