[PATCH v8 0/4] arm64: mte: allow async MTE to be upgraded to sync on a per-CPU basis

[PATCH v8 0/4] arm64: mte: allow async MTE to be upgraded to sync on a per-CPU basis

[Peter Collingbourne]

On some CPUs the performance of MTE in synchronous mode is similar
to that of asynchronous mode. This makes it worthwhile to enable
synchronous mode on those CPUs when asynchronous mode is requested,
in order to gain the error detection benefits of synchronous mode
without the performance downsides. Therefore, make it possible for
user programs to opt into upgrading to synchronous mode on those CPUs.

This is done by introducing a notion of a preferred TCF mode, which is
controlled on a per-CPU basis by a sysfs node. The existing SYNC and
ASYNC TCF settings are repurposed as bitfields that specify a set of
possible modes. If the preferred TCF mode for a particular CPU is in
the user-provided mode set (this will always be the case for mode sets
containing more than one mode because the kernel only supports two tag
checking modes, but future kernels may support more modes) then that
mode is used when running on that CPU, otherwise one of the modes in
the task's mode set will be selected in a currently unspecified manner.

v8:
- split into multiple patches
- remove MTE_CTRL_TCF_NONE
- improve documentation
- disable preemption and add comment to mte_update_sctlr_user
- bring back PR_MTE_TCF_SHIFT for source compatibility
- address formatting nit

v7:
- switch to new API proposed on list

v6:
- switch to strings in sysfs nodes instead of TCF values

v5:
- updated documentation
- address some nits in mte.c

v4:
- switch to new mte_ctrl field
- make register_mte_upgrade_async_sysctl return an int
- change the sysctl to take 0 or 1 instead of raw TCF values
- "same as" -> "similar to"

v3:
- drop the device tree support
- add documentation
- add static_assert to ensure no overlap with real HW bits
- move per-CPU variable initialization to mte.c
- use smp_call_function_single instead of stop_machine

v2:
- make it an opt-in behavior
- change the format of the device tree node
- also allow controlling the feature via sysfs

Peter Collingbourne (4):
  arm64: mte: rename gcr_user_excl to mte_ctrl
  arm64: mte: change ASYNC and SYNC TCF settings into bitfields
  arm64: mte: introduce a per-CPU tag checking mode preference
  Documentation: document the preferred tag checking mode feature

 .../arm64/memory-tagging-extension.rst        |  48 +++++-
 arch/arm64/include/asm/mte.h                  |   4 +
 arch/arm64/include/asm/processor.h            |   8 +-
 arch/arm64/kernel/asm-offsets.c               |   2 +-
 arch/arm64/kernel/entry.S                     |   4 +-
 arch/arm64/kernel/mte.c                       | 159 ++++++++++++------
 include/uapi/linux/prctl.h                    |  11 +-
 7 files changed, 171 insertions(+), 65 deletions(-)

-- 
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[PATCH v8 1/4] arm64: mte: rename gcr_user_excl to mte_ctrl

[Peter Collingbourne]

We are going to use this field to store more data. To prepare for
that, rename it and change the users to rely on the bit position of
gcr_user_excl in mte_ctrl.

Link: https://linux-review.googlesource.com/id/Ie1fd18e480100655f5d22137f5b22f4f3a9f9e2e
Signed-off-by: Peter Collingbourne <pcc@google.com>
---
 arch/arm64/include/asm/processor.h |  5 ++++-
 arch/arm64/kernel/asm-offsets.c    |  2 +-
 arch/arm64/kernel/entry.S          |  4 ++--
 arch/arm64/kernel/mte.c            | 14 ++++++++------
 4 files changed, 15 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index 9df3feeee890..6322fb1714d5 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -16,6 +16,9 @@
  */
 #define NET_IP_ALIGN	0
 
+#define MTE_CTRL_GCR_USER_EXCL_SHIFT	0
+#define MTE_CTRL_GCR_USER_EXCL_MASK	0xffff
+
 #ifndef __ASSEMBLY__
 
 #include <linux/build_bug.h>
@@ -151,7 +154,7 @@ struct thread_struct {
 	struct ptrauth_keys_kernel	keys_kernel;
 #endif
 #ifdef CONFIG_ARM64_MTE
-	u64			gcr_user_excl;
+	u64			mte_ctrl;
 #endif
 	u64			sctlr_user;
 };
diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
index 0cb34ccb6e73..63d02cd67b44 100644
--- a/arch/arm64/kernel/asm-offsets.c
+++ b/arch/arm64/kernel/asm-offsets.c
@@ -49,7 +49,7 @@ int main(void)
   DEFINE(THREAD_KEYS_KERNEL,	offsetof(struct task_struct, thread.keys_kernel));
 #endif
 #ifdef CONFIG_ARM64_MTE
-  DEFINE(THREAD_GCR_EL1_USER,	offsetof(struct task_struct, thread.gcr_user_excl));
+  DEFINE(THREAD_MTE_CTRL,	offsetof(struct task_struct, thread.mte_ctrl));
 #endif
   BLANK();
   DEFINE(S_X0,			offsetof(struct pt_regs, regs[0]));
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 3513984a88bd..ce59280355c5 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -182,7 +182,7 @@ alternative_else_nop_endif
 	 * the RRND (bit[16]) setting.
 	 */
 	mrs_s	\tmp2, SYS_GCR_EL1
-	bfi	\tmp2, \tmp, #0, #16
+	bfxil	\tmp2, \tmp, #MTE_CTRL_GCR_USER_EXCL_SHIFT, #16
 	msr_s	SYS_GCR_EL1, \tmp2
 #endif
 	.endm
@@ -205,7 +205,7 @@ alternative_else_nop_endif
 alternative_if_not ARM64_MTE
 	b	1f
 alternative_else_nop_endif
-	ldr	\tmp, [\tsk, #THREAD_GCR_EL1_USER]
+	ldr	\tmp, [\tsk, #THREAD_MTE_CTRL]
 
 	mte_set_gcr \tmp, \tmp2
 1:
diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 125a10e413e9..d3884d09513d 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -199,7 +199,7 @@ static void update_gcr_el1_excl(u64 excl)
 
 static void set_gcr_el1_excl(u64 excl)
 {
-	current->thread.gcr_user_excl = excl;
+	current->thread.mte_ctrl = excl;
 
 	/*
 	 * SYS_GCR_EL1 will be set to current->thread.gcr_user_excl value
@@ -263,8 +263,8 @@ void mte_suspend_exit(void)
 long set_mte_ctrl(struct task_struct *task, unsigned long arg)
 {
 	u64 sctlr = task->thread.sctlr_user & ~SCTLR_EL1_TCF0_MASK;
-	u64 gcr_excl = ~((arg & PR_MTE_TAG_MASK) >> PR_MTE_TAG_SHIFT) &
-		       SYS_GCR_EL1_EXCL_MASK;
+	u64 mte_ctrl = (~((arg & PR_MTE_TAG_MASK) >> PR_MTE_TAG_SHIFT) &
+			SYS_GCR_EL1_EXCL_MASK) << MTE_CTRL_GCR_USER_EXCL_SHIFT;
 
 	if (!system_supports_mte())
 		return 0;
@@ -285,10 +285,10 @@ long set_mte_ctrl(struct task_struct *task, unsigned long arg)
 
 	if (task != current) {
 		task->thread.sctlr_user = sctlr;
-		task->thread.gcr_user_excl = gcr_excl;
+		task->thread.mte_ctrl = mte_ctrl;
 	} else {
 		set_task_sctlr_el1(sctlr);
-		set_gcr_el1_excl(gcr_excl);
+		set_gcr_el1_excl(mte_ctrl);
 	}
 
 	return 0;
@@ -297,7 +297,9 @@ long set_mte_ctrl(struct task_struct *task, unsigned long arg)
 long get_mte_ctrl(struct task_struct *task)
 {
 	unsigned long ret;
-	u64 incl = ~task->thread.gcr_user_excl & SYS_GCR_EL1_EXCL_MASK;
+	u64 mte_ctrl = task->thread.mte_ctrl;
+	u64 incl = (~mte_ctrl >> MTE_CTRL_GCR_USER_EXCL_SHIFT) &
+		   SYS_GCR_EL1_EXCL_MASK;
 
 	if (!system_supports_mte())
 		return 0;
-- 
2.32.0.93.g670b81a890-goog


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[PATCH v8 2/4] arm64: mte: change ASYNC and SYNC TCF settings into bitfields

[Peter Collingbourne]

Allow the user program to specify both ASYNC and SYNC TCF modes by
repurposing the existing constants as bitfields. This will allow the
kernel to select one of the modes on behalf of the user program. With
this patch the kernel will always select async mode, but a subsequent
patch will make this configurable.

Link: https://linux-review.googlesource.com/id/Icc5923c85a8ea284588cc399ae74fd19ec291230
Signed-off-by: Peter Collingbourne <pcc@google.com>
---
 arch/arm64/include/asm/mte.h       |  4 ++
 arch/arm64/include/asm/processor.h |  3 ++
 arch/arm64/kernel/mte.c            | 78 ++++++++++++------------------
 include/uapi/linux/prctl.h         | 11 +++--
 4 files changed, 45 insertions(+), 51 deletions(-)

diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
index bc88a1ced0d7..719687412798 100644
--- a/arch/arm64/include/asm/mte.h
+++ b/arch/arm64/include/asm/mte.h
@@ -40,6 +40,7 @@ void mte_free_tag_storage(char *storage);
 void mte_sync_tags(pte_t *ptep, pte_t pte);
 void mte_copy_page_tags(void *kto, const void *kfrom);
 void mte_thread_init_user(void);
+void mte_update_sctlr_user(struct task_struct *task);
 void mte_thread_switch(struct task_struct *next);
 void mte_suspend_enter(void);
 void mte_suspend_exit(void);
@@ -62,6 +63,9 @@ static inline void mte_copy_page_tags(void *kto, const void *kfrom)
 static inline void mte_thread_init_user(void)
 {
 }
+static inline void mte_update_sctlr_user(struct task_struct *task)
+{
+}
 static inline void mte_thread_switch(struct task_struct *next)
 {
 }
diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index 6322fb1714d5..80ceb9cbdd60 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -19,6 +19,9 @@
 #define MTE_CTRL_GCR_USER_EXCL_SHIFT	0
 #define MTE_CTRL_GCR_USER_EXCL_MASK	0xffff
 
+#define MTE_CTRL_TCF_SYNC		(1UL << 16)
+#define MTE_CTRL_TCF_ASYNC		(1UL << 17)
+
 #ifndef __ASSEMBLY__
 
 #include <linux/build_bug.h>
diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index d3884d09513d..7b8c241be24e 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -197,16 +197,6 @@ static void update_gcr_el1_excl(u64 excl)
 	sysreg_clear_set_s(SYS_GCR_EL1, SYS_GCR_EL1_EXCL_MASK, excl);
 }
 
-static void set_gcr_el1_excl(u64 excl)
-{
-	current->thread.mte_ctrl = excl;
-
-	/*
-	 * SYS_GCR_EL1 will be set to current->thread.gcr_user_excl value
-	 * by mte_set_user_gcr() in kernel_exit,
-	 */
-}
-
 void mte_thread_init_user(void)
 {
 	if (!system_supports_mte())
@@ -216,15 +206,31 @@ void mte_thread_init_user(void)
 	dsb(ish);
 	write_sysreg_s(0, SYS_TFSRE0_EL1);
 	clear_thread_flag(TIF_MTE_ASYNC_FAULT);
-	/* disable tag checking */
-	set_task_sctlr_el1((current->thread.sctlr_user & ~SCTLR_EL1_TCF0_MASK) |
-			   SCTLR_EL1_TCF0_NONE);
-	/* reset tag generation mask */
-	set_gcr_el1_excl(SYS_GCR_EL1_EXCL_MASK);
+	/* disable tag checking and reset tag generation mask */
+	current->thread.mte_ctrl = MTE_CTRL_GCR_USER_EXCL_MASK;
+	mte_update_sctlr_user(current);
+	set_task_sctlr_el1(current->thread.sctlr_user);
+}
+
+void mte_update_sctlr_user(struct task_struct *task)
+{
+	unsigned long sctlr = task->thread.sctlr_user;
+	unsigned long pref = MTE_CTRL_TCF_ASYNC;
+	unsigned long mte_ctrl = task->thread.mte_ctrl;
+	unsigned long resolved_mte_tcf = (mte_ctrl & pref) ? pref : mte_ctrl;
+
+	sctlr &= ~SCTLR_EL1_TCF0_MASK;
+	if (resolved_mte_tcf & MTE_CTRL_TCF_ASYNC)
+		sctlr |= SCTLR_EL1_TCF0_ASYNC;
+	else if (resolved_mte_tcf & MTE_CTRL_TCF_SYNC)
+		sctlr |= SCTLR_EL1_TCF0_SYNC;
+	task->thread.sctlr_user = sctlr;
 }
 
 void mte_thread_switch(struct task_struct *next)
 {
+	mte_update_sctlr_user(next);
+
 	/*
 	 * Check if an async tag exception occurred at EL1.
 	 *
@@ -262,33 +268,21 @@ void mte_suspend_exit(void)
 
 long set_mte_ctrl(struct task_struct *task, unsigned long arg)
 {
-	u64 sctlr = task->thread.sctlr_user & ~SCTLR_EL1_TCF0_MASK;
 	u64 mte_ctrl = (~((arg & PR_MTE_TAG_MASK) >> PR_MTE_TAG_SHIFT) &
 			SYS_GCR_EL1_EXCL_MASK) << MTE_CTRL_GCR_USER_EXCL_SHIFT;
 
 	if (!system_supports_mte())
 		return 0;
 
-	switch (arg & PR_MTE_TCF_MASK) {
-	case PR_MTE_TCF_NONE:
-		sctlr |= SCTLR_EL1_TCF0_NONE;
-		break;
-	case PR_MTE_TCF_SYNC:
-		sctlr |= SCTLR_EL1_TCF0_SYNC;
-		break;
-	case PR_MTE_TCF_ASYNC:
-		sctlr |= SCTLR_EL1_TCF0_ASYNC;
-		break;
-	default:
-		return -EINVAL;
-	}
+	if (arg & PR_MTE_TCF_ASYNC)
+		mte_ctrl |= MTE_CTRL_TCF_ASYNC;
+	if (arg & PR_MTE_TCF_SYNC)
+		mte_ctrl |= MTE_CTRL_TCF_SYNC;
 
-	if (task != current) {
-		task->thread.sctlr_user = sctlr;
-		task->thread.mte_ctrl = mte_ctrl;
-	} else {
-		set_task_sctlr_el1(sctlr);
-		set_gcr_el1_excl(mte_ctrl);
+	task->thread.mte_ctrl = mte_ctrl;
+	if (task == current) {
+		mte_update_sctlr_user(task);
+		set_task_sctlr_el1(task->thread.sctlr_user);
 	}
 
 	return 0;
@@ -305,18 +299,10 @@ long get_mte_ctrl(struct task_struct *task)
 		return 0;
 
 	ret = incl << PR_MTE_TAG_SHIFT;
-
-	switch (task->thread.sctlr_user & SCTLR_EL1_TCF0_MASK) {
-	case SCTLR_EL1_TCF0_NONE:
-		ret |= PR_MTE_TCF_NONE;
-		break;
-	case SCTLR_EL1_TCF0_SYNC:
-		ret |= PR_MTE_TCF_SYNC;
-		break;
-	case SCTLR_EL1_TCF0_ASYNC:
+	if (mte_ctrl & MTE_CTRL_TCF_ASYNC)
 		ret |= PR_MTE_TCF_ASYNC;
-		break;
-	}
+	if (mte_ctrl & MTE_CTRL_TCF_SYNC)
+		ret |= PR_MTE_TCF_SYNC;
 
 	return ret;
 }
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 18a9f59dc067..d3a5afb4c1ae 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -234,14 +234,15 @@ struct prctl_mm_map {
 #define PR_GET_TAGGED_ADDR_CTRL		56
 # define PR_TAGGED_ADDR_ENABLE		(1UL << 0)
 /* MTE tag check fault modes */
-# define PR_MTE_TCF_SHIFT		1
-# define PR_MTE_TCF_NONE		(0UL << PR_MTE_TCF_SHIFT)
-# define PR_MTE_TCF_SYNC		(1UL << PR_MTE_TCF_SHIFT)
-# define PR_MTE_TCF_ASYNC		(2UL << PR_MTE_TCF_SHIFT)
-# define PR_MTE_TCF_MASK		(3UL << PR_MTE_TCF_SHIFT)
+# define PR_MTE_TCF_NONE		0
+# define PR_MTE_TCF_SYNC		(1UL << 1)
+# define PR_MTE_TCF_ASYNC		(1UL << 2)
+# define PR_MTE_TCF_MASK		(PR_MTE_TCF_SYNC | PR_MTE_TCF_ASYNC)
 /* MTE tag inclusion mask */
 # define PR_MTE_TAG_SHIFT		3
 # define PR_MTE_TAG_MASK		(0xffffUL << PR_MTE_TAG_SHIFT)
+/* Unused; kept only for source compatibility */
+# define PR_MTE_TCF_SHIFT		1
 
 /* Control reclaim behavior when allocating memory */
 #define PR_SET_IO_FLUSHER		57
-- 
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[PATCH v8 3/4] arm64: mte: introduce a per-CPU tag checking mode preference

[Peter Collingbourne]

Add a per-CPU sysfs node, mte_tcf_preferred, that allows the preferred
tag checking mode to be configured. The current possible values are
async and sync.

Link: https://linux-review.googlesource.com/id/I7493dcd533a2785a1437b16c3f6b50919f840854
Signed-off-by: Peter Collingbourne <pcc@google.com>
---
 arch/arm64/kernel/mte.c | 77 +++++++++++++++++++++++++++++++++++++++--
 1 file changed, 75 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 7b8c241be24e..9c82e27b30f9 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -4,6 +4,7 @@
  */
 
 #include <linux/bitops.h>
+#include <linux/cpu.h>
 #include <linux/kernel.h>
 #include <linux/mm.h>
 #include <linux/prctl.h>
@@ -26,6 +27,8 @@ u64 gcr_kernel_excl __ro_after_init;
 
 static bool report_fault_once = true;
 
+static DEFINE_PER_CPU_READ_MOSTLY(u64, mte_tcf_preferred);
+
 #ifdef CONFIG_KASAN_HW_TAGS
 /* Whether the MTE asynchronous mode is enabled. */
 DEFINE_STATIC_KEY_FALSE(mte_async_mode);
@@ -214,17 +217,24 @@ void mte_thread_init_user(void)
 
 void mte_update_sctlr_user(struct task_struct *task)
 {
+	/*
+	 * This can only be called on the current or next task since the CPU
+	 * must match where the thread is going to run.
+	 */
 	unsigned long sctlr = task->thread.sctlr_user;
-	unsigned long pref = MTE_CTRL_TCF_ASYNC;
 	unsigned long mte_ctrl = task->thread.mte_ctrl;
-	unsigned long resolved_mte_tcf = (mte_ctrl & pref) ? pref : mte_ctrl;
+	unsigned long pref, resolved_mte_tcf;
 
+	preempt_disable();
+	pref = __this_cpu_read(mte_tcf_preferred);
+	resolved_mte_tcf = (mte_ctrl & pref) ? pref : mte_ctrl;
 	sctlr &= ~SCTLR_EL1_TCF0_MASK;
 	if (resolved_mte_tcf & MTE_CTRL_TCF_ASYNC)
 		sctlr |= SCTLR_EL1_TCF0_ASYNC;
 	else if (resolved_mte_tcf & MTE_CTRL_TCF_SYNC)
 		sctlr |= SCTLR_EL1_TCF0_SYNC;
 	task->thread.sctlr_user = sctlr;
+	preempt_enable();
 }
 
 void mte_thread_switch(struct task_struct *next)
@@ -441,3 +451,66 @@ int mte_ptrace_copy_tags(struct task_struct *child, long request,
 
 	return ret;
 }
+
+static ssize_t mte_tcf_preferred_show(struct device *dev,
+				      struct device_attribute *attr, char *buf)
+{
+	switch (per_cpu(mte_tcf_preferred, dev->id)) {
+	case MTE_CTRL_TCF_ASYNC:
+		return sysfs_emit(buf, "async\n");
+	case MTE_CTRL_TCF_SYNC:
+		return sysfs_emit(buf, "sync\n");
+	default:
+		return sysfs_emit(buf, "???\n");
+	}
+}
+
+static void sync_sctlr(void *arg)
+{
+	mte_update_sctlr_user(current);
+	set_task_sctlr_el1(current->thread.sctlr_user);
+}
+
+static ssize_t mte_tcf_preferred_store(struct device *dev,
+				       struct device_attribute *attr,
+				       const char *buf, size_t count)
+{
+	ssize_t ret = 0;
+	u64 tcf;
+
+	if (sysfs_streq(buf, "async"))
+		tcf = MTE_CTRL_TCF_ASYNC;
+	else if (sysfs_streq(buf, "sync"))
+		tcf = MTE_CTRL_TCF_SYNC;
+	else
+		return -EINVAL;
+
+	device_lock(dev);
+	per_cpu(mte_tcf_preferred, dev->id) = tcf;
+
+	if (cpu_online(dev->id))
+		ret = smp_call_function_single(dev->id, sync_sctlr, NULL, 0);
+	if (ret == 0)
+		ret = count;
+	device_unlock(dev);
+
+	return ret;
+}
+static DEVICE_ATTR_RW(mte_tcf_preferred);
+
+static int register_mte_tcf_preferred_sysctl(void)
+{
+	unsigned int cpu;
+
+	if (!system_supports_mte())
+		return 0;
+
+	for_each_possible_cpu(cpu) {
+		per_cpu(mte_tcf_preferred, cpu) = MTE_CTRL_TCF_ASYNC;
+		device_create_file(get_cpu_device(cpu),
+				   &dev_attr_mte_tcf_preferred);
+	}
+
+	return 0;
+}
+subsys_initcall(register_mte_tcf_preferred_sysctl);
-- 
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[PATCH v8 4/4] Documentation: document the preferred tag checking mode feature

[Peter Collingbourne]

Document the functionality added in the previous patches.

Link: https://linux-review.googlesource.com/id/I48217cc3e8b8da33abc08cbaddc11cf4360a1b86
Signed-off-by: Peter Collingbourne <pcc@google.com>
---
 .../arm64/memory-tagging-extension.rst        | 48 ++++++++++++++++---
 1 file changed, 41 insertions(+), 7 deletions(-)

diff --git a/Documentation/arm64/memory-tagging-extension.rst b/Documentation/arm64/memory-tagging-extension.rst
index b540178a93f8..7b99c8f428eb 100644
--- a/Documentation/arm64/memory-tagging-extension.rst
+++ b/Documentation/arm64/memory-tagging-extension.rst
@@ -77,14 +77,20 @@ configurable behaviours:
   address is unknown).
 
 The user can select the above modes, per thread, using the
-``prctl(PR_SET_TAGGED_ADDR_CTRL, flags, 0, 0, 0)`` system call where
-``flags`` contain one of the following values in the ``PR_MTE_TCF_MASK``
+``prctl(PR_SET_TAGGED_ADDR_CTRL, flags, 0, 0, 0)`` system call where ``flags``
+contains any number of the following values in the ``PR_MTE_TCF_MASK``
 bit-field:
 
-- ``PR_MTE_TCF_NONE``  - *Ignore* tag check faults
+- ``PR_MTE_TCF_NONE``  - *Ignore* tag check faults
+                         (ignored if combined with other options)
 - ``PR_MTE_TCF_SYNC``  - *Synchronous* tag check fault mode
 - ``PR_MTE_TCF_ASYNC`` - *Asynchronous* tag check fault mode
 
+If no modes are specified, tag check faults are ignored. If a single
+mode is specified, the program will run in that mode. If multiple
+modes are specified, the mode is selected as described in the "Per-CPU
+preferred tag checking modes" section below.
+
 The current tag check fault mode can be read using the
 ``prctl(PR_GET_TAGGED_ADDR_CTRL, 0, 0, 0, 0)`` system call.
 
@@ -120,13 +126,39 @@ in the ``PR_MTE_TAG_MASK`` bit-field.
 interface provides an include mask. An include mask of ``0`` (exclusion
 mask ``0xffff``) results in the CPU always generating tag ``0``.
 
+Per-CPU preferred tag checking mode
+-----------------------------------
+
+On some CPUs the performance of MTE in stricter tag checking modes
+is similar to that of less strict tag checking modes. This makes it
+worthwhile to enable stricter checks on those CPUs when a less strict
+checking mode is requested, in order to gain the error detection
+benefits of the stricter checks without the performance downsides. To
+support this scenario, a privileged user may configure a stricter
+tag checking mode as the CPU's preferred tag checking mode.
+
+The preferred tag checking mode for each CPU is controlled by
+``/sys/devices/system/cpu/cpu<N>/mte_tcf_preferred``, to which a
+privileged user may write the value ``async`` or ``sync``.  The default
+preferred mode for each CPU is ``async``.
+
+To allow a program to potentially run in the CPU's preferred tag
+checking mode, the user program may set multiple tag check fault mode
+bits in the ``flags`` argument to the ``prctl(PR_SET_TAGGED_ADDR_CTRL,
+flags, 0, 0, 0)`` system call. If the CPU's preferred tag checking
+mode is in the task's set of provided tag checking modes (this will
+always be the case at present because the kernel only supports two
+tag checking modes, but future kernels may support more modes), that
+mode will be selected. Otherwise, one of the modes in the task's mode
+set will be selected in a currently unspecified manner.
+
 Initial process state
 ---------------------
 
 On ``execve()``, the new process has the following configuration:
 
 - ``PR_TAGGED_ADDR_ENABLE`` set to 0 (disabled)
-- Tag checking mode set to ``PR_MTE_TCF_NONE``
+- No tag checking modes are selected (tag check faults ignored)
 - ``PR_MTE_TAG_MASK`` set to 0 (all tags excluded)
 - ``PSTATE.TCO`` set to 0
 - ``PROT_MTE`` not set on any of the initial memory maps
@@ -251,11 +283,13 @@ Example of correct usage
                     return EXIT_FAILURE;
 
             /*
-             * Enable the tagged address ABI, synchronous MTE tag check faults and
-             * allow all non-zero tags in the randomly generated set.
+             * Enable the tagged address ABI, synchronous or asynchronous MTE
+             * tag check faults (based on per-CPU preference) and allow all
+             * non-zero tags in the randomly generated set.
              */
             if (prctl(PR_SET_TAGGED_ADDR_CTRL,
-                      PR_TAGGED_ADDR_ENABLE | PR_MTE_TCF_SYNC | (0xfffe << PR_MTE_TAG_SHIFT),
+                      PR_TAGGED_ADDR_ENABLE | PR_MTE_TCF_SYNC | PR_MTE_TCF_ASYNC |
+                      (0xfffe << PR_MTE_TAG_SHIFT),
                       0, 0, 0)) {
                     perror("prctl() failed");
                     return EXIT_FAILURE;
-- 
2.32.0.93.g670b81a890-goog


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Re: [PATCH v8 4/4] Documentation: document the preferred tag checking mode feature

[Catalin Marinas]

On Wed, Jun 30, 2021 at 04:15:09PM -0700, Peter Collingbourne wrote:
> +Per-CPU preferred tag checking mode
> +-----------------------------------
> +
> +On some CPUs the performance of MTE in stricter tag checking modes
> +is similar to that of less strict tag checking modes. This makes it
> +worthwhile to enable stricter checks on those CPUs when a less strict
> +checking mode is requested, in order to gain the error detection
> +benefits of the stricter checks without the performance downsides. To
> +support this scenario, a privileged user may configure a stricter
> +tag checking mode as the CPU's preferred tag checking mode.
> +
> +The preferred tag checking mode for each CPU is controlled by
> +``/sys/devices/system/cpu/cpu<N>/mte_tcf_preferred``, to which a
> +privileged user may write the value ``async`` or ``sync``.  The default
> +preferred mode for each CPU is ``async``.
> +
> +To allow a program to potentially run in the CPU's preferred tag
> +checking mode, the user program may set multiple tag check fault mode
> +bits in the ``flags`` argument to the ``prctl(PR_SET_TAGGED_ADDR_CTRL,
> +flags, 0, 0, 0)`` system call. If the CPU's preferred tag checking
> +mode is in the task's set of provided tag checking modes (this will
> +always be the case at present because the kernel only supports two
> +tag checking modes, but future kernels may support more modes), that
> +mode will be selected. Otherwise, one of the modes in the task's mode
> +set will be selected in a currently unspecified manner.

I'm ok with leaving this undefined (it's in the spirit of many
architecture features ;)). In practice we may choose async or asym over
sync but the user shouldn't care if it passed multiple flags.

-- 
Catalin

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Re: [PATCH v8 1/4] arm64: mte: rename gcr_user_excl to mte_ctrl

[Catalin Marinas]

On Wed, Jun 30, 2021 at 04:15:06PM -0700, Peter Collingbourne wrote:
> We are going to use this field to store more data. To prepare for
> that, rename it and change the users to rely on the bit position of
> gcr_user_excl in mte_ctrl.
> 
> Link: https://linux-review.googlesource.com/id/Ie1fd18e480100655f5d22137f5b22f4f3a9f9e2e
> Signed-off-by: Peter Collingbourne <pcc@google.com>

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

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Re: [PATCH v8 2/4] arm64: mte: change ASYNC and SYNC TCF settings into bitfields

[Catalin Marinas]

On Wed, Jun 30, 2021 at 04:15:07PM -0700, Peter Collingbourne wrote:
> Allow the user program to specify both ASYNC and SYNC TCF modes by
> repurposing the existing constants as bitfields. This will allow the
> kernel to select one of the modes on behalf of the user program. With
> this patch the kernel will always select async mode, but a subsequent
> patch will make this configurable.
> 
> Link: https://linux-review.googlesource.com/id/Icc5923c85a8ea284588cc399ae74fd19ec291230
> Signed-off-by: Peter Collingbourne <pcc@google.com>

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

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Re: [PATCH v8 3/4] arm64: mte: introduce a per-CPU tag checking mode preference

[Catalin Marinas]

On Wed, Jun 30, 2021 at 04:15:08PM -0700, Peter Collingbourne wrote:
> Add a per-CPU sysfs node, mte_tcf_preferred, that allows the preferred
> tag checking mode to be configured. The current possible values are
> async and sync.
> 
> Link: https://linux-review.googlesource.com/id/I7493dcd533a2785a1437b16c3f6b50919f840854
> Signed-off-by: Peter Collingbourne <pcc@google.com>

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

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Re: [PATCH v8 4/4] Documentation: document the preferred tag checking mode feature

[Catalin Marinas]

On Wed, Jun 30, 2021 at 04:15:09PM -0700, Peter Collingbourne wrote:
> Document the functionality added in the previous patches.
> 
> Link: https://linux-review.googlesource.com/id/I48217cc3e8b8da33abc08cbaddc11cf4360a1b86
> Signed-off-by: Peter Collingbourne <pcc@google.com>

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

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Re: [PATCH v8 0/4] arm64: mte: allow async MTE to be upgraded to sync on a per-CPU basis

[Catalin Marinas]

On Wed, Jun 30, 2021 at 04:15:05PM -0700, Peter Collingbourne wrote:
> On some CPUs the performance of MTE in synchronous mode is similar
> to that of asynchronous mode. This makes it worthwhile to enable
> synchronous mode on those CPUs when asynchronous mode is requested,
> in order to gain the error detection benefits of synchronous mode
> without the performance downsides. Therefore, make it possible for
> user programs to opt into upgrading to synchronous mode on those CPUs.
> 
> This is done by introducing a notion of a preferred TCF mode, which is
> controlled on a per-CPU basis by a sysfs node. The existing SYNC and
> ASYNC TCF settings are repurposed as bitfields that specify a set of
> possible modes. If the preferred TCF mode for a particular CPU is in
> the user-provided mode set (this will always be the case for mode sets
> containing more than one mode because the kernel only supports two tag
> checking modes, but future kernels may support more modes) then that
> mode is used when running on that CPU, otherwise one of the modes in
> the task's mode set will be selected in a currently unspecified manner.

The series looks good to me but please post it again after -rc1 if it
doesn't apply cleanly.

Thanks.

-- 
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Re: [PATCH v8 0/4] arm64: mte: allow async MTE to be upgraded to sync on a per-CPU basis

[Peter Collingbourne]

On Thu, Jul 1, 2021 at 10:10 AM Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> On Wed, Jun 30, 2021 at 04:15:05PM -0700, Peter Collingbourne wrote:
> > On some CPUs the performance of MTE in synchronous mode is similar
> > to that of asynchronous mode. This makes it worthwhile to enable
> > synchronous mode on those CPUs when asynchronous mode is requested,
> > in order to gain the error detection benefits of synchronous mode
> > without the performance downsides. Therefore, make it possible for
> > user programs to opt into upgrading to synchronous mode on those CPUs.
> >
> > This is done by introducing a notion of a preferred TCF mode, which is
> > controlled on a per-CPU basis by a sysfs node. The existing SYNC and
> > ASYNC TCF settings are repurposed as bitfields that specify a set of
> > possible modes. If the preferred TCF mode for a particular CPU is in
> > the user-provided mode set (this will always be the case for mode sets
> > containing more than one mode because the kernel only supports two tag
> > checking modes, but future kernels may support more modes) then that
> > mode is used when running on that CPU, otherwise one of the modes in
> > the task's mode set will be selected in a currently unspecified manner.
>
> The series looks good to me but please post it again after -rc1 if it
> doesn't apply cleanly.

Thanks. I tried applying this series to linux-next and it applied
cleanly, so it seems likely that it will apply cleanly to rc1. I will
let you know if that is not the case though.

I received feedback elsewhere that we should be adding documentation
under Documentation/ABI for the new sysfs node. Also while developing
my GCR on task switch patch I noticed a small cleanup that could be
made to patch 2 of this series. I went ahead and made both of those
improvements in v9.

Peter

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