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

[Peter Collingbourne]

On Tue, Feb 15, 2022 at 2:51 PM Will Deacon <will at kernel.org> wrote:
>
> 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

Sorry for the trouble, I sent a v5 with the fix.

Peter