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Date:	Mon, 10 Nov 2014 15:55:35 -0800
From:	Andi Kleen <andi@...stfloor.org>
To:	x86@...nel.org
Cc:	linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>
Subject: [PATCH 4/8] x86: Add support for rd/wr fs/gs base

From: Andi Kleen <ak@...ux.intel.com>

Introduction:

IvyBridge added four new instructions to directly write the fs and gs
64bit base registers. Previously this had to be done with a system
call to write to MSRs. The main use case is fast user space threading
and switching the fs/gs registers quickly there. Another use
case is having (relatively) cheap access to a new address
register per thread.

The instructions are opt-in and have to be explicitely enabled
by the OS.

For more details on how to use the instructions see
Documentation/x86/fsgs.txt added in a followon patch.

Paranoid exception path changes:
===============================

The paranoid entry/exit code is used for any NMI like
exception.

Previously Linux couldn't support the new instructions
because the paranoid entry code relied on the gs base never being
negative outside the kernel to decide when to use swaps. It would
check the gs MSR value and assume it was already running in
kernel if negative.

To get rid of this assumption we have to revamp the paranoid exception
path to not rely on this. We can use the new instructions
to get (relatively) quick access to the GS value, and use
it directly.

This is also significantly faster than a MSR read, so will speed
NMIs (critical for profiling)

The kernel gs for the paranoid path is now stored at the
bottom of the IST stack (so that it can be derived from RSP).
For this we need to know the size of the IST stack
(4K or 8K), which is now passed in as a stack parameter
to save_paranoid.

The original patch compared the gs with the kernel gs and
assumed that if it was identical, swapgs was not needed
(and no user space processing was needed). This
was nice and simple and didn't need a lot of changes.

But this had the side effect that if a user process set its
GS to the same as the kernel it may lose rescheduling
checks (so a racing reschedule IPI would have been
only acted upon the next non paranoid interrupt)

This version now switches to full save/restore of the GS.
This requires quite some changes in the paranoid path.
Unfortunately I didn't come up with a simpler scheme.

Previously we had a flag in EBX that indicated whether
SWAPGS needs to be called later or not. In the new scheme
this turns into a tristate, with a new "restore from R15"
mode that is used when the FSGSBASE instructions are available.
In this case the GS base is saved and restored.
The exit paths are all adjusted to handle this correctly.

There is one complication: to allow debuggers (especially
from the int3 or debug vectors) access to the user GS
we need to save it in the task struct. Normally
the next context switch would overwrite it with the wrong
value from kernel_gs, so we set new flag also in task_struct
that prevents it. The flag is cleared on context switch.

[Even with the additional flag the new FS/GS context switch
is vastly faster than the old MSR based version for bases >4GB]

To prevent recursive interrupts clobbering this
state in the task_struct this is only done for interrupts
coming directly from ring 3.

After a reschedule comes back we check if the flag is still
set. If it wasn't set the GS is back in the (swapped) kernel
gs so we revert to the SWAPGS mode, instead of restoring GS.

So the three possible states for the paranoid exit path are:

- Do nothing (pre FSGSBASE), if we interrupted the kernel
as determined by the existing negative GS
- Do swapgs, if we interrupted user space with positive GS
(pre FSGSBASE), or we saved gs, but it was overwritten
later by a context switch (with FSGSBASE)
- Restore from R15 (with FSGSBASE), if the gs base was saved
in R15, and not overwritten by a context switch.

Context switch changes:
======================

Then after these changes we need to also use the new instructions
to save/restore fs and gs, so that the new values set by the
users won't disappear.  This is also significantly
faster for the case when the 64bit base has to be switched
(that is when GS is larger than 4GB), as we can replace
the slow MSR write with a faster wr[fg]sbase execution.

The instructions do not context switch
the segment index, so the old invariant that fs or gs index
have to be 0 for a different 64bit value to stick is still
true. Previously it was enforced by arch_prctl, now the user
program has to make sure it keeps the segment indexes zero.
If it doesn't the changes may not stick.

[Yes, this implies that programs can find out when they
got context switched. However they could already do that
before by checking the timing.]

This is in term enables fast switching when there are
enough threads that their TLS segment does not fit below 4GB,
or alternatively programs that use fs as an additional base
register will not get a sigificant context switch penalty.

It is all done in a single patch because there was no
simple way to do it in pieces without having crash
holes inbetween.

v2: Change to save/restore GS instead of using swapgs
based on the value. Large scale changes.
v3: Fix wrong flag initialization in fallback path.
Thanks 0day!
Signed-off-by: Andi Kleen <ak@...ux.intel.com>
---
 arch/x86/include/asm/processor.h |   4 +
 arch/x86/kernel/asm-offsets_64.c |   4 +
 arch/x86/kernel/cpu/common.c     |   6 ++
 arch/x86/kernel/entry_64.S       | 163 +++++++++++++++++++++++++++++++--------
 arch/x86/kernel/process_64.c     |  31 +++++++-
 5 files changed, 170 insertions(+), 38 deletions(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index eb71ec7..987a631 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -522,6 +522,10 @@ struct thread_struct {
 	 * a short time
 	 */
 	unsigned char fpu_counter;
+	/*
+	 * Set to one when gs is already saved.
+	 */
+	unsigned char  gs_saved;
 };
 
 /*
diff --git a/arch/x86/kernel/asm-offsets_64.c b/arch/x86/kernel/asm-offsets_64.c
index e7c798b..32ab510 100644
--- a/arch/x86/kernel/asm-offsets_64.c
+++ b/arch/x86/kernel/asm-offsets_64.c
@@ -85,6 +85,10 @@ int main(void)
 
 	DEFINE(__NR_ia32_syscall_max, sizeof(syscalls_ia32) - 1);
 	DEFINE(IA32_NR_syscalls, sizeof(syscalls_ia32));
+	BLANK();
+
+	OFFSET(task_thread_gs, task_struct, thread.gs);
+	OFFSET(task_thread_gs_saved, task_struct, thread.gs_saved);
 
 	return 0;
 }
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 59def4c..db74af5 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -953,6 +953,9 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 #ifdef CONFIG_NUMA
 	numa_add_cpu(smp_processor_id());
 #endif
+
+	if (cpu_has(c, X86_FEATURE_FSGSBASE))
+		set_in_cr4(X86_CR4_FSGSBASE);
 }
 
 #ifdef CONFIG_X86_64
@@ -1351,10 +1354,13 @@ void cpu_init(void)
 	 */
 	if (!oist->ist[0]) {
 		char *estacks = per_cpu(exception_stacks, cpu);
+		void *gs = per_cpu(irq_stack_union.gs_base, cpu);
 
 		for (v = 0; v < N_EXCEPTION_STACKS; v++) {
 			if (v == DEBUG_STACK - 1)
 				estacks = per_cpu(debug_stack, cpu);
+			/* Store GS at bottom of stack for bootstrap access */
+			*(void **)estacks = gs;
 			estacks += exception_stack_sizes[v];
 			oist->ist[v] = t->x86_tss.ist[v] =
 					(unsigned long)estacks;
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index df088bb..9f7fde5 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -58,6 +58,8 @@
 #include <asm/context_tracking.h>
 #include <asm/smap.h>
 #include <asm/pgtable_types.h>
+#include <asm/alternative-asm.h>
+#include <asm/fsgs.h>
 #include <linux/err.h>
 
 /* Avoid __ASSEMBLER__'ifying <linux/audit.h> just for this.  */
@@ -283,32 +285,86 @@ ENDPROC(native_usergs_sysret64)
 	TRACE_IRQS_OFF
 	.endm
 
+/* Values of the ebx flag: */
+#define DO_RESTORE_R15	 2 /* Restore GS at end */
+#define DO_SWAPGS	 1 /* Use SWAPGS at end */
+#define DO_NOTHING	 0 /* Back to ring 0 with same gs */
+
+/*
+ * Stack layout:
+ * +16 pt_regs
+ * +8  stack mask for ist or 0
+ * +0  return address.
+ */
+#define OFF 16
+#define STKMSK 8
+
 ENTRY(save_paranoid)
-	XCPT_FRAME 1 RDI+8
+	XCPT_FRAME 1 RDI+OFF
 	cld
-	movq %rdi, RDI+8(%rsp)
-	movq %rsi, RSI+8(%rsp)
-	movq_cfi rdx, RDX+8
-	movq_cfi rcx, RCX+8
-	movq_cfi rax, RAX+8
-	movq %r8, R8+8(%rsp)
-	movq %r9, R9+8(%rsp)
-	movq %r10, R10+8(%rsp)
-	movq %r11, R11+8(%rsp)
-	movq_cfi rbx, RBX+8
-	movq %rbp, RBP+8(%rsp)
-	movq %r12, R12+8(%rsp)
-	movq %r13, R13+8(%rsp)
-	movq %r14, R14+8(%rsp)
-	movq %r15, R15+8(%rsp)
-	movl $1,%ebx
+	movq %rdi, RDI+OFF(%rsp)
+	movq %rsi, RSI+OFF(%rsp)
+	movq_cfi rdx, RDX+OFF
+	movq_cfi rcx, RCX+OFF
+	movq_cfi rax, RAX+OFF
+	movq %r8, R8+OFF(%rsp)
+	movq %r9, R9+OFF(%rsp)
+	movq %r10, R10+OFF(%rsp)
+	movq %r11, R11+OFF(%rsp)
+	movq_cfi rbx, RBX+OFF
+	movq %rbp, RBP+OFF(%rsp)
+	movq %r12, R12+OFF(%rsp)
+	movq %r13, R13+OFF(%rsp)
+	movq %r14, R14+OFF(%rsp)
+	movq %r15, R15+OFF(%rsp)
+	movq $-1,ORIG_RAX+OFF(%rsp)	/* no syscall to restart */
+33:
+	ASM_NOP5	/* May be replaced with jump to paranoid_save_gs */
+34:
+	movl $DO_NOTHING,%ebx
 	movl $MSR_GS_BASE,%ecx
 	rdmsr
 	testl %edx,%edx
 	js 1f	/* negative -> in kernel */
 	SWAPGS
-	xorl %ebx,%ebx
+	movl $DO_SWAPGS,%ebx
 1:	ret
+
+	/* Patch in jump to paranoid_save_gs for X86_FEATURE_FSGSBASE */
+	.section .altinstr_replacement,"ax"
+35:	.byte 0xe9 /* 32bit near jump */
+	.long paranoid_save_gs-34b
+	.previous
+	.section .altinstructions,"a"
+	altinstruction_entry 33b,35b,X86_FEATURE_FSGSBASE,5,5
+	.previous
+
+	/* Faster version not using RDMSR, and also not assuming
+	 * anything about the previous GS value.
+	 * This allows the user to arbitarily change GS using
+	 * WRGSBASE.
+	 */
+paranoid_save_gs:
+	RDGSBASE_R15			# read previous gs
+	movq STKMSK(%rsp),%rax		# get ist stack mask
+	andq %rsp,%rax			# get bottom of stack
+	movq (%rax),%rdi		# get expected GS
+	WRGSBASE_RDI			# set gs for kernel
+	mov $DO_RESTORE_R15,%ebx	# flag for exit code
+	testl $3,CS+OFF(%rsp)		# did we come from ring 0?
+	je paranoid_save_gs_kernel
+	/*
+	 * Saving GS in the task struct allows a debugger to manipulate
+	 * it. We only do this when coming from ring 3 to avoid recursion
+	 * clobbering the state.
+	 */
+	movq PER_CPU_VAR(current_task),%rcx # get current
+	movb $1,task_thread_gs_saved(%rcx)  # tell context switch gs is
+					    # is already saved
+	movq %r15,task_thread_gs(%rcx)      # save gs in task struct
+paranoid_save_gs_kernel:
+	ret
+
 	CFI_ENDPROC
 END(save_paranoid)
 
@@ -1040,7 +1096,8 @@ apicinterrupt IRQ_WORK_VECTOR \
  */
 #define INIT_TSS_IST(x) PER_CPU_VAR(init_tss) + (TSS_ist + ((x) - 1) * 8)
 
-.macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1
+.macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1 \
+		stack_mask=-EXCEPTION_STKSZ
 ENTRY(\sym)
 	/* Sanity check */
 	.if \shift_ist != -1 && \paranoid == 0
@@ -1064,7 +1121,10 @@ ENTRY(\sym)
 	CFI_ADJUST_CFA_OFFSET ORIG_RAX-R15
 
 	.if \paranoid
+	pushq_cfi $\stack_mask		/* ist stack size */
 	call save_paranoid
+	/* r15: previous gs */
+	popq_cfi %rax			/* Drop ist stack size */
 	.else
 	call error_entry
 	.endif
@@ -1099,7 +1159,7 @@ ENTRY(\sym)
 	.endif
 
 	.if \paranoid
-	jmp paranoid_exit		/* %ebx: no swapgs flag */
+	jmp paranoid_exit		/* %ebx: no swapgs flag, r15: old gs */
 	.else
 	jmp error_exit			/* %ebx: no swapgs flag */
 	.endif
@@ -1287,8 +1347,10 @@ apicinterrupt3 HYPERVISOR_CALLBACK_VECTOR \
 	hyperv_callback_vector hyperv_vector_handler
 #endif /* CONFIG_HYPERV */
 
-idtentry debug do_debug has_error_code=0 paranoid=1 shift_ist=DEBUG_STACK
-idtentry int3 do_int3 has_error_code=0 paranoid=1 shift_ist=DEBUG_STACK
+idtentry debug do_debug has_error_code=0 paranoid=1 shift_ist=DEBUG_STACK \
+	 stack_mask=-DEBUG_STKSZ
+idtentry int3 do_int3 has_error_code=0 paranoid=1 shift_ist=DEBUG_STACK \
+	 stack_mask=-DEBUG_STKSZ
 idtentry stack_segment do_stack_segment has_error_code=1 paranoid=1
 #ifdef CONFIG_XEN
 idtentry xen_debug do_debug has_error_code=0
@@ -1317,49 +1379,69 @@ idtentry machine_check has_error_code=0 paranoid=1 do_sym=*machine_check_vector(
 	 * hard flags at once, atomically)
 	 */
 
-	/* ebx:	no swapgs flag */
+	/* ebx:	no swapgs flag, r15: gs if ebx == DO_RESTORE_R15 */
 ENTRY(paranoid_exit)
 	DEFAULT_FRAME
 	DISABLE_INTERRUPTS(CLBR_NONE)
 	TRACE_IRQS_OFF_DEBUG
-	testl %ebx,%ebx				/* swapgs needed? */
-	jnz paranoid_restore
+	cmpl  $DO_NOTHING,%ebx		/* swapgs needed? */
+	je  paranoid_restore
 	testl $3,CS(%rsp)
 	jnz   paranoid_userspace
 paranoid_swapgs:
 	TRACE_IRQS_IRETQ 0
+	cmpl  $DO_RESTORE_R15,%ebx	/* restore gs? */
+	je  paranoid_restore_gs
 	SWAPGS_UNSAFE_STACK
 	RESTORE_ALL 8
 	jmp irq_return
+paranoid_restore_gs:
+	WRGSBASE_R15
+	RESTORE_ALL 8
+	jmp irq_return
 paranoid_restore:
 	TRACE_IRQS_IRETQ_DEBUG 0
 	RESTORE_ALL 8
 	jmp irq_return
 paranoid_userspace:
 	GET_THREAD_INFO(%rcx)
-	movl TI_flags(%rcx),%ebx
-	andl $_TIF_WORK_MASK,%ebx
-	jz paranoid_swapgs
+	movl TI_flags(%rcx),%ebp
+	andl $_TIF_WORK_MASK,%ebp
+	jz paranoid_clear_gs_flag
 	movq %rsp,%rdi			/* &pt_regs */
 	call sync_regs
 	movq %rax,%rsp			/* switch stack for scheduling */
-	testl $_TIF_NEED_RESCHED,%ebx
+	testl $_TIF_NEED_RESCHED,%ebp
 	jnz paranoid_schedule
-	movl %ebx,%edx			/* arg3: thread flags */
+	movl %ebp,%edx			/* arg3: thread flags */
 	TRACE_IRQS_ON
 	ENABLE_INTERRUPTS(CLBR_NONE)
 	xorl %esi,%esi 			/* arg2: oldset */
 	movq %rsp,%rdi 			/* arg1: &pt_regs */
 	call do_notify_resume
-	DISABLE_INTERRUPTS(CLBR_NONE)
-	TRACE_IRQS_OFF
-	jmp paranoid_userspace
+	jmp paranoid_after_schedule
+paranoid_clear_gs_flag:
+	movq PER_CPU_VAR(current_task),%rcx
+	movb $0,task_thread_gs_saved(%rcx)
+	jmp paranoid_swapgs
 paranoid_schedule:
 	TRACE_IRQS_ON
 	ENABLE_INTERRUPTS(CLBR_ANY)
 	SCHEDULE_USER
+paranoid_after_schedule:
 	DISABLE_INTERRUPTS(CLBR_ANY)
 	TRACE_IRQS_OFF
+	cmpl $DO_RESTORE_R15,%ebx
+	jne  paranoid_userspace
+	movq PER_CPU_VAR(current_task),%rcx
+	cmpb $0,task_thread_gs_saved(%rcx) /* did we schedule? */
+	jne  paranoid_userspace		   /* did not schedule: keep our gs */
+	/*
+	 * Otherwise the context switch has put the correct gs into
+	 * kernel_gs, so we can just end with swapgs and it
+	 * will DTRT.
+	 */
+	mov  $DO_SWAPGS,%ebx		   /* force swapgs after schedule */
 	jmp paranoid_userspace
 	CFI_ENDPROC
 END(paranoid_exit)
@@ -1650,6 +1732,7 @@ end_repeat_nmi:
 	pushq_cfi $-1		/* ORIG_RAX: no syscall to restart */
 	subq $ORIG_RAX-R15, %rsp
 	CFI_ADJUST_CFA_OFFSET ORIG_RAX-R15
+	pushq_cfi $-EXCEPTION_STKSZ	/* ist stack size */
 	/*
 	 * Use save_paranoid to handle SWAPGS, but no need to use paranoid_exit
 	 * as we should not be calling schedule in NMI context.
@@ -1658,6 +1741,8 @@ end_repeat_nmi:
 	 * exceptions might do.
 	 */
 	call save_paranoid
+	/* r15: previous gs */
+	popq_cfi %rax		/* pop ist size */
 	DEFAULT_FRAME 0
 
 	/*
@@ -1682,11 +1767,21 @@ end_repeat_nmi:
 	je 1f
 	movq %r12, %cr2
 1:
-	
+	cmpl $DO_RESTORE_R15,%ebx
+	je nmi_gs_restore
 	testl %ebx,%ebx				/* swapgs needed? */
 	jnz nmi_restore
 nmi_swapgs:
 	SWAPGS_UNSAFE_STACK
+	jmp nmi_restore
+nmi_gs_restore:
+	/* If we came from ring 3 clear the saved gs flag. */
+	testl $3,CS(%rsp)
+	je   nmi_wrgsbase
+	movq PER_CPU_VAR(current_task),%rcx
+	movb $0,task_thread_gs_saved(%rcx)
+nmi_wrgsbase:
+	WRGSBASE_R15				/* restore gs */
 nmi_restore:
 	/* Pop the extra iret frame at once */
 	RESTORE_ALL 6*8
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 3ed4a68..df554e2 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -49,6 +49,7 @@
 #include <asm/syscalls.h>
 #include <asm/debugreg.h>
 #include <asm/switch_to.h>
+#include <asm/fsgs.h>
 
 asmlinkage extern void ret_from_fork(void);
 
@@ -308,6 +309,16 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 	 */
 	savesegment(fs, fsindex);
 	savesegment(gs, gsindex);
+	if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
+		prev->fs = rdfsbase();
+		/* Interrupts are disabled here. */
+		if (!prev->gs_saved) {
+			swapgs();
+			prev->gs = rdgsbase();
+			swapgs();
+		}
+		prev->gs_saved = 0;
+	}
 
 	load_TLS(next, cpu);
 
@@ -338,8 +349,12 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 			prev->fs = 0;
 	}
 	/* when next process has a 64bit base use it */
-	if (next->fs)
-		wrmsrl(MSR_FS_BASE, next->fs);
+	if (next->fs) {
+		if (static_cpu_has(X86_FEATURE_FSGSBASE))
+			wrfsbase(next->fs);
+		else
+			wrmsrl(MSR_FS_BASE, next->fs);
+	}
 	prev->fsindex = fsindex;
 
 	if (unlikely(gsindex | next->gsindex | prev->gs)) {
@@ -347,8 +362,16 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 		if (gsindex)
 			prev->gs = 0;
 	}
-	if (next->gs)
-		wrmsrl(MSR_KERNEL_GS_BASE, next->gs);
+	if (next->gs) {
+		if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
+			/* Interrupts are disabled here. */
+			swapgs();
+			wrgsbase(next->gs);
+			swapgs();
+		} else {
+			wrmsrl(MSR_KERNEL_GS_BASE, next->gs);
+		}
+	}
 	prev->gsindex = gsindex;
 
 	switch_fpu_finish(next_p, fpu);
-- 
1.9.3

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