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-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:
-
-http://lkml.kernel.org/r/<20110529191055.GC9835%40elte.hu>
-
-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.
-
- - 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
- SWAPGS
-
-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 */
- SWAPGS
- 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.