[PATCH v4] arm64: mte: avoid clearing PSTATE.TCO on entry unless necessary

[Will Deacon]

On Mon, Jan 24, 2022 at 04:58:50PM -0800, Peter Collingbourne wrote:
> On some microarchitectures, clearing PSTATE.TCO is expensive. Clearing
> TCO is only necessary if in-kernel MTE is enabled, or if MTE is
> enabled in the userspace process in synchronous (or, soon, asymmetric)
> mode, because we do not report uaccess faults to userspace in none
> or asynchronous modes. Therefore, adjust the kernel entry code to
> clear TCO only if necessary.
> 
> Because it is now possible to switch to a task in which TCO needs to
> be clear from a task in which TCO is set, we also need to do the same
> thing on task switch.
> 
> Signed-off-by: Peter Collingbourne <pcc at google.com>
> Link: https://linux-review.googlesource.com/id/I52d82a580bd0500d420be501af2c35fa8c90729e
> Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>
> ---
> v4:
> - some changes suggested by Catalin
> 
> v3:
> - switch to a C implementation
> 
> v2:
> - do the same thing in cpu_switch_to()
> 
>  arch/arm64/include/asm/mte.h     | 22 ++++++++++++++++++++++
>  arch/arm64/kernel/entry-common.c |  3 +++
>  arch/arm64/kernel/entry.S        |  7 -------
>  arch/arm64/kernel/mte.c          |  3 +++
>  4 files changed, 28 insertions(+), 7 deletions(-)

I applied this, but it broke the 'allmodconfig' build so I had to drop it:

 | In file included from ./arch/arm64/include/asm/pgtable.h:12,
 |                  from ./include/linux/pgtable.h:6,
 |                  from ./include/linux/kasan.h:30,
 |                  from ./include/linux/slab.h:136,
 |                  from ./include/linux/resource_ext.h:11,
 |                  from ./include/linux/acpi.h:14,
 |                  from ./include/acpi/apei.h:9,
 |                  from ./include/acpi/ghes.h:5,
 |                  from ./include/linux/arm_sdei.h:8,
 |                  from arch/arm64/kernel/asm-offsets.c:10:
 | ./arch/arm64/include/asm/mte.h: In function ‘mte_disable_tco_entry’:
 | ./arch/arm64/include/asm/mte.h:106:6: error: implicit declaration of function ‘kasan_hw_tags_enabled’ [-Werror=implicit-function-declaration]
 |   106 |  if (kasan_hw_tags_enabled() ||
 |       |      ^~~~~~~~~~~~~~~~~~~~~
 | In file included from ./include/linux/slab.h:136,
 |                  from ./include/linux/resource_ext.h:11,
 |                  from ./include/linux/acpi.h:14,
 |                  from ./include/acpi/apei.h:9,
 |                  from ./include/acpi/ghes.h:5,
 |                  from ./include/linux/arm_sdei.h:8,
 |                  from arch/arm64/kernel/asm-offsets.c:10:
 | ./include/linux/kasan.h: At top level:
 | ./include/linux/kasan.h:108:20: error: conflicting types for ‘kasan_hw_tags_enabled’
 |   108 | static inline bool kasan_hw_tags_enabled(void)
 |       |                    ^~~~~~~~~~~~~~~~~~~~~
 | In file included from ./arch/arm64/include/asm/pgtable.h:12,
 |                  from ./include/linux/pgtable.h:6,
 |                  from ./include/linux/kasan.h:30,
 |                  from ./include/linux/slab.h:136,
 |                  from ./include/linux/resource_ext.h:11,
 |                  from ./include/linux/acpi.h:14,
 |                  from ./include/acpi/apei.h:9,
 |                  from ./include/acpi/ghes.h:5,
 |                  from ./include/linux/arm_sdei.h:8,
 |                  from arch/arm64/kernel/asm-offsets.c:10:
 | ./arch/arm64/include/asm/mte.h:106:6: note: previous implicit declaration of ‘kasan_hw_tags_enabled’ was here
 |   106 |  if (kasan_hw_tags_enabled() ||
 |       |      ^~~~~~~~~~~~~~~~~~~~~
 | cc1: all warnings being treated as errors
 | make[1]: *** [scripts/Makefile.build:121: arch/arm64/kernel/asm-offsets.s] Error 1
 | make: *** [Makefile:1191: prepare0] Error 2

Will