1.. SPDX-License-Identifier: GPL-2.0
  2
  3==============
  4Kernel Entries
  5==============
  6
  7This file documents some of the kernel entries in
  8arch/x86/entry/entry_64.S.  A lot of this explanation is adapted from
  9an email from Ingo Molnar:
 10
 11https://lore.kernel.org/r/20110529191055.GC9835%40elte.hu
 12
 13The x86 architecture has quite a few different ways to jump into
 14kernel code.  Most of these entry points are registered in
 15arch/x86/kernel/traps.c and implemented in arch/x86/entry/entry_64.S
 16for 64-bit, arch/x86/entry/entry_32.S for 32-bit and finally
 17arch/x86/entry/entry_64_compat.S which implements the 32-bit compatibility
 18syscall entry points and thus provides for 32-bit processes the
 19ability to execute syscalls when running on 64-bit kernels.
 20
 21The IDT vector assignments are listed in arch/x86/include/asm/irq_vectors.h.
 22
 23Some of these entries are:
 24
 25 - system_call: syscall instruction from 64-bit code.
 26
 27 - entry_INT80_compat: int 0x80 from 32-bit or 64-bit code; compat syscall
 28   either way.
 29
 30 - entry_INT80_compat, ia32_sysenter: syscall and sysenter from 32-bit
 31   code
 32
 33 - interrupt: An array of entries.  Every IDT vector that doesn't
 34   explicitly point somewhere else gets set to the corresponding
 35   value in interrupts.  These point to a whole array of
 36   magically-generated functions that make their way to common_interrupt()
 37   with the interrupt number as a parameter.
 38
 39 - APIC interrupts: Various special-purpose interrupts for things
 40   like TLB shootdown.
 41
 42 - Architecturally-defined exceptions like divide_error.
 43
 44There are a few complexities here.  The different x86-64 entries
 45have different calling conventions.  The syscall and sysenter
 46instructions have their own peculiar calling conventions.  Some of
 47the IDT entries push an error code onto the stack; others don't.
 48IDT entries using the IST alternative stack mechanism need their own
 49magic to get the stack frames right.  (You can find some
 50documentation in the AMD APM, Volume 2, Chapter 8 and the Intel SDM,
 51Volume 3, Chapter 6.)
 52
 53Dealing with the swapgs instruction is especially tricky.  Swapgs
 54toggles whether gs is the kernel gs or the user gs.  The swapgs
 55instruction is rather fragile: it must nest perfectly and only in
 56single depth, it should only be used if entering from user mode to
 57kernel mode and then when returning to user-space, and precisely
 58so. If we mess that up even slightly, we crash.
 59
 60So when we have a secondary entry, already in kernel mode, we *must
 61not* use SWAPGS blindly - nor must we forget doing a SWAPGS when it's
 62not switched/swapped yet.
 63
 64Now, there's a secondary complication: there's a cheap way to test
 65which mode the CPU is in and an expensive way.
 66
 67The cheap way is to pick this info off the entry frame on the kernel
 68stack, from the CS of the ptregs area of the kernel stack::
 69
 70	xorl %ebx,%ebx
 71	testl $3,CS+8(%rsp)
 72	je error_kernelspace
 73	SWAPGS
 74
 75The expensive (paranoid) way is to read back the MSR_GS_BASE value
 76(which is what SWAPGS modifies)::
 77
 78	movl $1,%ebx
 79	movl $MSR_GS_BASE,%ecx
 80	rdmsr
 81	testl %edx,%edx
 82	js 1f   /* negative -> in kernel */
 83	SWAPGS
 84	xorl %ebx,%ebx
 85  1:	ret
 86
 87If we are at an interrupt or user-trap/gate-alike boundary then we can
 88use the faster check: the stack will be a reliable indicator of
 89whether SWAPGS was already done: if we see that we are a secondary
 90entry interrupting kernel mode execution, then we know that the GS
 91base has already been switched. If it says that we interrupted
 92user-space execution then we must do the SWAPGS.
 93
 94But if we are in an NMI/MCE/DEBUG/whatever super-atomic entry context,
 95which might have triggered right after a normal entry wrote CS to the
 96stack but before we executed SWAPGS, then the only safe way to check
 97for GS is the slower method: the RDMSR.
 98
 99Therefore, super-atomic entries (except NMI, which is handled separately)
100must use idtentry with paranoid=1 to handle gsbase correctly.  This
101triggers three main behavior changes:
102
103 - Interrupt entry will use the slower gsbase check.
104 - Interrupt entry from user mode will switch off the IST stack.
105 - Interrupt exit to kernel mode will not attempt to reschedule.
106
107We try to only use IST entries and the paranoid entry code for vectors
108that absolutely need the more expensive check for the GS base - and we
109generate all 'normal' entry points with the regular (faster) paranoid=0
110variant.