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Subject[PATCH v5 2/9] x86-64: Document some of entry_64.S
Signed-off-by: Andy Lutomirski <>
Documentation/x86/entry_64.txt | 98 ++++++++++++++++++++++++++++++++++++++++
arch/x86/kernel/entry_64.S | 2 +
2 files changed, 100 insertions(+), 0 deletions(-)
create mode 100644 Documentation/x86/entry_64.txt

diff --git a/Documentation/x86/entry_64.txt b/Documentation/x86/entry_64.txt
new file mode 100644
index 0000000..7869f14
--- /dev/null
+++ b/Documentation/x86/entry_64.txt
@@ -0,0 +1,98 @@
+This file documents some of the kernel entries in
+arch/x86/kernel/entry_64.S. A lot of this explanation is adapted from
+an email from Ingo Molnar:
+The x86 architecture has quite a few different ways to jump into
+kernel code. Most of these entry points are registered in
+arch/x86/kernel/traps.c and implemented in arch/x86/kernel/entry_64.S
+and arch/x86/ia32/ia32entry.S.
+The IDT vector assignments are listed in arch/x86/include/irq_vectors.h.
+Some of these entries are:
+ - system_call: syscall instruction from 64-bit code.
+ - ia32_syscall: int 0x80 from 32-bit or 64-bit code; compat syscall
+ either way.
+ - ia32_syscall, ia32_sysenter: syscall and sysenter from 32-bit
+ code
+ - interrupt: An array of entries. Every IDT vector that doesn't
+ explicitly point somewhere else gets set to the corresponding
+ value in interrupts. These point to a whole array of
+ magically-generated functions that make their way to do_IRQ with
+ the interrupt number as a parameter.
+ - emulate_vsyscall: int 0xcc, a special non-ABI entry used by
+ vsyscall emulation.
+ - APIC interrupts: Various special-purpose interrupts for things
+ like TLB shootdown.
+ - Architecturally-defined exceptions like divide_error.
+There are a few complexities here. The different x86-64 entries
+have different calling conventions. The syscall and sysenter
+instructions have their own peculiar calling conventions. Some of
+the IDT entries push an error code onto the stack; others don't.
+IDT entries using the IST alternative stack mechanism need their own
+magic to get the stack frames right. (You can find some
+documentation in the AMD APM, Volume 2, Chapter 8 and the Intel SDM,
+Volume 3, Chapter 6.)
+Dealing with the swapgs instruction is especially tricky. Swapgs
+toggles whether gs is the kernel gs or the user gs. The swapgs
+instruction is rather fragile: it must nest perfectly and only in
+single depth, it should only be used if entering from user mode to
+kernel mode and then when returning to user-space, and precisely
+so. If we mess that up even slightly, we crash.
+So when we have a secondary entry, already in kernel mode, we *must
+not* use SWAPGS blindly - nor must we forget doing a SWAPGS when it's
+not switched/swapped yet.
+Now, there's a secondary complication: there's a cheap way to test
+which mode the CPU is in and an expensive way.
+The cheap way is to pick this info off the entry frame on the kernel
+stack, from the CS of the ptregs area of the kernel stack:
+ xorl %ebx,%ebx
+ testl $3,CS+8(%rsp)
+ je error_kernelspace
+The expensive (paranoid) way is to read back the MSR_GS_BASE value
+(which is what SWAPGS modifies):
+ movl $1,%ebx
+ movl $MSR_GS_BASE,%ecx
+ rdmsr
+ testl %edx,%edx
+ js 1f /* negative -> in kernel */
+ xorl %ebx,%ebx
+1: ret
+and the whole paranoid non-paranoid macro complexity is about whether
+to suffer that RDMSR cost.
+If we are at an interrupt or user-trap/gate-alike boundary then we can
+use the faster check: the stack will be a reliable indicator of
+whether SWAPGS was already done: if we see that we are a secondary
+entry interrupting kernel mode execution, then we know that the GS
+base has already been switched. If it says that we interrupted
+user-space execution then we must do the SWAPGS.
+But if we are in an NMI/MCE/DEBUG/whatever super-atomic entry context,
+which might have triggered right after a normal entry wrote CS to the
+stack but before we executed SWAPGS, then the only safe way to check
+for GS is the slower method: the RDMSR.
+So we try only to mark those entry methods 'paranoid' that absolutely
+need the more expensive check for the GS base - and we generate all
+'normal' entry points with the regular (faster) entry macros.
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 8a445a0..72c4a77 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -9,6 +9,8 @@
* entry.S contains the system-call and fault low-level handling routines.
+ * Some of this is documented in Documentation/x86/entry_64.txt
+ *
* NOTE: This code handles signal-recognition, which happens every time
* after an interrupt and after each system call.

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