On 2/9/21 11:35 AM, Catalin Marinas wrote:

On Mon, Feb 08, 2021 at 04:56:14PM +0000, Vincenzo Frascino wrote:

quoted

diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
index 0deb88467111..f43d78aee593 100644
--- a/arch/arm64/include/asm/uaccess.h
+++ b/arch/arm64/include/asm/uaccess.h

@@ -188,6 +188,21 @@ static inline void __uaccess_enable_tco(void)
 				 ARM64_MTE, CONFIG_KASAN_HW_TAGS));
 }
 
+/* Whether the MTE asynchronous mode is enabled. */
+DECLARE_STATIC_KEY_FALSE(mte_async_mode);
+
+static inline void __uaccess_disable_tco_async(void)
+{
+	if (static_branch_unlikely(&mte_async_mode))
+		 __uaccess_disable_tco();
+}
+
+static inline void __uaccess_enable_tco_async(void)
+{
+	if (static_branch_unlikely(&mte_async_mode))
+		__uaccess_enable_tco();
+}

I would add a comment here along the lines of what's in the commit log:
these functions disable tag checking only if in MTE async mode since the
sync mode generates exceptions synchronously and the nofault or
load_unaligned_zeropad can handle them.

Good point, increases clarity. I will add it in the next version.

quoted

+
 static inline void uaccess_disable_privileged(void)
 {
 	__uaccess_disable_tco();

@@ -307,8 +322,10 @@ do {									\
 do {									\
 	int __gkn_err = 0;						\
 									\
+	__uaccess_enable_tco_async();					\
 	__raw_get_mem("ldr", *((type *)(dst)),				\
 		      (__force type *)(src), __gkn_err);		\
+	__uaccess_disable_tco_async();					\
 	if (unlikely(__gkn_err))					\
 		goto err_label;						\
 } while (0)

@@ -379,9 +396,11 @@ do {									\
 #define __put_kernel_nofault(dst, src, type, err_label)			\
 do {									\
 	int __pkn_err = 0;						\
+	__uaccess_enable_tco_async();					\
 									\

Nitpick: for consistency with the __get_kernel_nofault() function,
please move the empty line above __uaccess_enable_tco_async().

Ok, will do in the next version.

quoted

 	__raw_put_mem("str", *((type *)(src)),				\
 		      (__force type *)(dst), __pkn_err);		\
+	__uaccess_disable_tco_async();					\
 	if (unlikely(__pkn_err))					\
 		goto err_label;						\
 } while(0)

[...]

quoted

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 92078e1eb627..60531afc706e 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c

@@ -27,6 +27,10 @@ u64 gcr_kernel_excl __ro_after_init;
 
 static bool report_fault_once = true;
 
+/* Whether the MTE asynchronous mode is enabled. */
+DEFINE_STATIC_KEY_FALSE(mte_async_mode);
+EXPORT_SYMBOL_GPL(mte_async_mode);
+
 static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
 {
 	pte_t old_pte = READ_ONCE(*ptep);

@@ -170,6 +174,12 @@ void mte_enable_kernel_sync(void)
 void mte_enable_kernel_async(void)
 {
 	__mte_enable_kernel("asynchronous", SCTLR_ELx_TCF_ASYNC);
+
+	/*
+	 * This function is called on each active smp core, we do not
+	 * to take cpu_hotplug_lock again.
+	 */
+	static_branch_enable_cpuslocked(&mte_async_mode);
 }

Do we need to disable mte_async_mode in mte_enable_kernel_sync()? I
think currently that's only done at boot time but kasan may gain some
run-time features and change the mode dynamically.

Indeed, as you are saying at the moment is done at boot only but it is better to
make the code more future proof. Maybe with a note in the commit message.

-- 
Regards,
Vincenzo

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