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Date: Sat, 13 Sep 2014 18:35:34 +0400
From: Konstantin Khlebnikov <koct9i@...il.com>
To: x86@...nel.org, linux-kernel@...r.kernel.org
Cc: Thomas Gleixner <tglx@...utronix.de>,
Andi Kleen <ak@...ux.intel.com>,
Ingo Molnar <mingo@...hat.com>,
Dmitry Vyukov <dvyukov@...gle.com>,
"H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH RFC] x86_64: per-cpu memory for user-space
This patch implements user-space per-cpu memory in the same manner as in
kernel-space: each cpu has its own %gs base address. On x86_64 %fs is used
for thread local storage, %gs usually is free.
User-space application cannot prevent preemption but x86 read-modify-write
operations are atomic against interrupts and context switches. Thus percpu
counters, ring-buffer cursors, per-cpu locks and other cool things might
be implemented in a very efficient way.
After this patch kernel recalculates %gs at each context switch.
This's implemented only via MSR_KERNEL_GS_BASE. Loading base via gdt
selector might be faster but it's much more complicated.
By the way, newer Intel cpus have even faster instructions for
changing %fs/%gs, but they are still not supported by the kernel.
Additional overhead is near to zero: this patch adds one extra multiplication
into __switch_to (only if gs is set by user-space and its base is above 4Gb):
if (next->gs)
- wrmsrl(MSR_KERNEL_GS_BASE, next->gs);
+ wrmsrl(MSR_KERNEL_GS_BASE, next->gs +
+ cpu * next->gs_cpu_stride);
Child inherits setup from parent at clone because it gets a copy of task_struct.
Changing %gs via any other interface (selector, ARCH_SET_GS) disables striping.
Interface:
int arch_prctl(ARCH_GET_GS_PERCPU, unsigned long arg[2]);
int arch_prctl(ARCH_SET_GS_PERCPU, unsigned long arg[2]);
arg[0] - base address for cpu0
arg[1] - stride to each next cpu
Error codes:
-EINVAL - not implemented (or ia32 compat)
-ENOENT - not configured (only for get)
-EFAULT - arg isn't addressable
-EPERM - base above addressable space (only for set)
-EOVERFLOW - stride too big for this base and count nr_cpus (only for set)
Signed-off-by: Konstantin Khlebnikov <koct9i@...il.com>
---
arch/x86/include/asm/processor.h | 1 +
arch/x86/include/uapi/asm/prctl.h | 2 ++
arch/x86/kernel/process_64.c | 39 ++++++++++++++++++++++++++++++++++++-
3 files changed, 41 insertions(+), 1 deletion(-)
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index eb71ec7..102c1f9 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -484,6 +484,7 @@ struct thread_struct {
#endif
#ifdef CONFIG_X86_64
unsigned long fs;
+ unsigned long gs_cpu_stride;
#endif
unsigned long gs;
/* Save middle states of ptrace breakpoints */
diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index 3ac5032..026bd39 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -5,5 +5,7 @@
#define ARCH_SET_FS 0x1002
#define ARCH_GET_FS 0x1003
#define ARCH_GET_GS 0x1004
+#define ARCH_SET_GS_PERCPU 0x1005
+#define ARCH_GET_GS_PERCPU 0x1006
#endif /* _ASM_X86_PRCTL_H */
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index ca5b02d..5e7af75 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -351,7 +351,8 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
prev->gs = 0;
}
if (next->gs)
- wrmsrl(MSR_KERNEL_GS_BASE, next->gs);
+ wrmsrl(MSR_KERNEL_GS_BASE, next->gs +
+ cpu * next->gs_cpu_stride);
prev->gsindex = gsindex;
switch_fpu_finish(next_p, fpu);
@@ -469,6 +470,7 @@ long do_arch_prctl(struct task_struct *task, int code, unsigned long addr)
if (addr >= TASK_SIZE_OF(task))
return -EPERM;
cpu = get_cpu();
+ task->thread.gs_cpu_stride = 0;
/* handle small bases via the GDT because that's faster to
switch. */
if (addr <= 0xffffffff) {
@@ -544,6 +546,41 @@ long do_arch_prctl(struct task_struct *task, int code, unsigned long addr)
ret = put_user(base, (unsigned long __user *)addr);
break;
}
+ case ARCH_GET_GS_PERCPU:
+ if (test_tsk_thread_flag(task, TIF_ADDR32))
+ return -EINVAL;
+ if (!task->thread.gs || !task->thread.gs_cpu_stride)
+ return -ENOENT;
+ ret = put_user(task->thread.gs,
+ (unsigned long __user *)addr);
+ if (!ret)
+ ret = put_user(task->thread.gs_cpu_stride,
+ ((unsigned long __user *)addr) + 1);
+ break;
+ case ARCH_SET_GS_PERCPU: {
+ unsigned long arg[2];
+
+ if (test_tsk_thread_flag(task, TIF_ADDR32))
+ return -EINVAL;
+ if (copy_from_user(arg, (void __user *)addr, sizeof(arg)))
+ return -EFAULT;
+ if (arg[0] >= TASK_SIZE_MAX)
+ return -EPERM;
+ if (arg[1] > (TASK_SIZE_MAX - arg[0]) / num_possible_cpus())
+ return -EOVERFLOW;
+
+ task->thread.gsindex = 0;
+ task->thread.gs = arg[0];
+ task->thread.gs_cpu_stride = arg[1];
+ if (doit) {
+ cpu = get_cpu();
+ load_gs_index(0);
+ ret = wrmsrl_safe(MSR_KERNEL_GS_BASE,
+ arg[0] + cpu * arg[1]);
+ put_cpu();
+ }
+ break;
+ }
default:
ret = -EINVAL;
--
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