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    SubjectRe: [PATCH v2 4/5] arm64: mte: Add asymmetric mode support
    On Mon, Oct 4, 2021 at 10:23 PM Vincenzo Frascino
    <vincenzo.frascino@arm.com> wrote:
    >
    > MTE provides an asymmetric mode for detecting tag exceptions. In
    > particular, when such a mode is present, the CPU triggers a fault
    > on a tag mismatch during a load operation and asynchronously updates
    > a register when a tag mismatch is detected during a store operation.
    >
    > Add support for MTE asymmetric mode.
    >
    > Note: If the CPU does not support MTE asymmetric mode the kernel falls
    > back on synchronous mode which is the default for kasan=on.
    >
    > Cc: Will Deacon <will@kernel.org>
    > Cc: Catalin Marinas <catalin.marinas@arm.com>
    > Cc: Andrey Konovalov <andreyknvl@gmail.com>
    > Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
    > Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
    > ---
    > arch/arm64/include/asm/memory.h | 1 +
    > arch/arm64/include/asm/mte-kasan.h | 5 +++++
    > arch/arm64/kernel/mte.c | 33 +++++++++++++++++++++++++++++-
    > 3 files changed, 38 insertions(+), 1 deletion(-)
    >
    > diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
    > index f1745a843414..1b9a1e242612 100644
    > --- a/arch/arm64/include/asm/memory.h
    > +++ b/arch/arm64/include/asm/memory.h
    > @@ -243,6 +243,7 @@ static inline const void *__tag_set(const void *addr, u8 tag)
    > #ifdef CONFIG_KASAN_HW_TAGS
    > #define arch_enable_tagging_sync() mte_enable_kernel_sync()
    > #define arch_enable_tagging_async() mte_enable_kernel_async()
    > +#define arch_enable_tagging_asymm() mte_enable_kernel_asymm()
    > #define arch_force_async_tag_fault() mte_check_tfsr_exit()
    > #define arch_get_random_tag() mte_get_random_tag()
    > #define arch_get_mem_tag(addr) mte_get_mem_tag(addr)
    > diff --git a/arch/arm64/include/asm/mte-kasan.h b/arch/arm64/include/asm/mte-kasan.h
    > index 22420e1f8c03..478b9bcf69ad 100644
    > --- a/arch/arm64/include/asm/mte-kasan.h
    > +++ b/arch/arm64/include/asm/mte-kasan.h
    > @@ -130,6 +130,7 @@ static inline void mte_set_mem_tag_range(void *addr, size_t size, u8 tag,
    >
    > void mte_enable_kernel_sync(void);
    > void mte_enable_kernel_async(void);
    > +void mte_enable_kernel_asymm(void);
    >
    > #else /* CONFIG_ARM64_MTE */
    >
    > @@ -161,6 +162,10 @@ static inline void mte_enable_kernel_async(void)
    > {
    > }
    >
    > +static inline void mte_enable_kernel_asymm(void)
    > +{
    > +}
    > +
    > #endif /* CONFIG_ARM64_MTE */
    >
    > #endif /* __ASSEMBLY__ */
    > diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
    > index e5e801bc5312..b6ad6b861c25 100644
    > --- a/arch/arm64/kernel/mte.c
    > +++ b/arch/arm64/kernel/mte.c
    > @@ -26,7 +26,12 @@
    > static DEFINE_PER_CPU_READ_MOSTLY(u64, mte_tcf_preferred);
    >
    > #ifdef CONFIG_KASAN_HW_TAGS
    > -/* Whether the MTE asynchronous mode is enabled. */
    > +/*
    > + * The MTE asynchronous and asymmetric mode have the same
    > + * behavior for the store operations.
    > + *
    > + * Whether the MTE asynchronous or asymmetric mode is enabled.
    > + */
    > DEFINE_STATIC_KEY_FALSE(mte_async_mode);
    > EXPORT_SYMBOL_GPL(mte_async_mode);
    > #endif
    > @@ -137,6 +142,32 @@ void mte_enable_kernel_async(void)
    > if (!system_uses_mte_async_mode())
    > static_branch_enable(&mte_async_mode);
    > }
    > +
    > +void mte_enable_kernel_asymm(void)
    > +{
    > + if (cpus_have_cap(ARM64_MTE_ASYMM)) {
    > + __mte_enable_kernel("asymmetric", SCTLR_ELx_TCF_ASYMM);
    > +
    > + /*
    > + * MTE asymm mode behaves as async mode for store
    > + * operations. The mode is set system wide by the
    > + * first PE that executes this function.
    > + *
    > + * Note: If in future KASAN acquires a runtime switching
    > + * mode in between sync and async, this strategy needs
    > + * to be reviewed.
    > + */
    > + if (!system_uses_mte_async_mode())
    > + static_branch_enable(&mte_async_mode);

    Using this variable and function here still looks confusing. Maybe
    naming the variable mte_async_or_asymm_mode? Or
    mte_async_fault_possible similarly to KASAN?


    > + } else {
    > + /*
    > + * If the CPU does not support MTE asymmetric mode the
    > + * kernel falls back on synchronous mode which is the
    > + * default for kasan=on.
    > + */
    > + mte_enable_kernel_sync();
    > + }
    > +}
    > #endif
    >
    > #ifdef CONFIG_KASAN_HW_TAGS
    > --
    > 2.33.0
    >

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