Commit 5d497509 authored by Will Deacon's avatar Will Deacon Committed by Russell King
Browse files

ARM: 7926/1: mm: flesh out and fix the comments in the ASID allocator 



The ASID allocator has to deal with some pretty horrible behaviours by
the CPU, so expand on some of the comments in there so I remember why
we can never allocate ASID zero to a userspace task.

Reviewed-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
parent a7a04105

arch/arm/mm/context.c

+10 −6
Original line number Original line Diff line number Diff line
@@ -36,8 +36,8 @@ 
 * The context ID is used by debuggers and trace logic, and

 * The context ID is used by debuggers and trace logic, and

 * should be unique within all running processes.

 * should be unique within all running processes.

 *

 *

 * In big endian operation, the two 32 bit words are swapped if accesed by

 * In big endian operation, the two 32 bit words are swapped if accessed

 * non 64-bit operations.

 * by non-64-bit operations.

 */

 */

#define ASID_FIRST_VERSION	(1ULL << ASID_BITS)

#define ASID_FIRST_VERSION	(1ULL << ASID_BITS)

#define NUM_USER_ASIDS		ASID_FIRST_VERSION

#define NUM_USER_ASIDS		ASID_FIRST_VERSION
@@ -195,8 +195,11 @@ static u64 new_context(struct mm_struct *mm, unsigned int cpu) 
		 * Allocate a free ASID. If we can't find one, take a

		 * Allocate a free ASID. If we can't find one, take a

		 * note of the currently active ASIDs and mark the TLBs

		 * note of the currently active ASIDs and mark the TLBs

		 * as requiring flushes. We always count from ASID #1,

		 * as requiring flushes. We always count from ASID #1,

		 * as we reserve ASID #0 to switch via TTBR0 and indicate

		 * as we reserve ASID #0 to switch via TTBR0 and to

		 * rollover events.

		 * avoid speculative page table walks from hitting in

		 * any partial walk caches, which could be populated

		 * from overlapping level-1 descriptors used to map both

		 * the module area and the userspace stack.

		 */

		 */

		asid = find_next_zero_bit(asid_map, NUM_USER_ASIDS, cur_idx);

		asid = find_next_zero_bit(asid_map, NUM_USER_ASIDS, cur_idx);

		if (asid == NUM_USER_ASIDS) {

		if (asid == NUM_USER_ASIDS) {
@@ -224,8 +227,9 @@ void check_and_switch_context(struct mm_struct *mm, struct task_struct *tsk) 
		__check_vmalloc_seq(mm);

		__check_vmalloc_seq(mm);





	/*

	/*

	 * Required during context switch to avoid speculative page table

	 * We cannot update the pgd and the ASID atomicly with classic

	 * walking with the wrong TTBR.

	 * MMU, so switch exclusively to global mappings to avoid

	 * speculative page table walking with the wrong TTBR.

	 */

	 */

	cpu_set_reserved_ttbr0();

	cpu_set_reserved_ttbr0();

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