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x86/entry: Improve system call entry comments

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Ingo suggested that the comments should explain when the various
entries are used.  This adds these explanations and improves other
parts of the comments.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/9524ecef7a295347294300045d08354d6a57c6e7.1457578375.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Andy Lutomirski
2016-03-09 19:00:35 -08:00
committed by Ingo Molnar
parent 392a62549f
commit fda57b2267
3 changed files with 128 additions and 28 deletions
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61
arch/x86/entry/entry_32.S
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@@ -307,6 +307,38 @@ ENTRY(xen_sysenter_target)
jmp sysenter_past_esp
#endif
/*
* 32-bit SYSENTER entry.
*
* 32-bit system calls through the vDSO's __kernel_vsyscall enter here
* if X86_FEATURE_SEP is available. This is the preferred system call
* entry on 32-bit systems.
*
* The SYSENTER instruction, in principle, should *only* occur in the
* vDSO. In practice, a small number of Android devices were shipped
* with a copy of Bionic that inlined a SYSENTER instruction. This
* never happened in any of Google's Bionic versions -- it only happened
* in a narrow range of Intel-provided versions.
*
* SYSENTER loads SS, ESP, CS, and EIP from previously programmed MSRs.
* IF and VM in RFLAGS are cleared (IOW: interrupts are off).
* SYSENTER does not save anything on the stack,
* and does not save old EIP (!!!), ESP, or EFLAGS.
*
* To avoid losing track of EFLAGS.VM (and thus potentially corrupting
* user and/or vm86 state), we explicitly disable the SYSENTER
* instruction in vm86 mode by reprogramming the MSRs.
*
* Arguments:
* eax system call number
* ebx arg1
* ecx arg2
* edx arg3
* esi arg4
* edi arg5
* ebp user stack
* 0(%ebp) arg6
*/
ENTRY(entry_SYSENTER_32)
movl TSS_sysenter_sp0(%esp), %esp
sysenter_past_esp:
@@ -397,7 +429,34 @@ sysenter_past_esp:
GLOBAL(__end_SYSENTER_singlestep_region)
ENDPROC(entry_SYSENTER_32)
# system call handler stub
/*
* 32-bit legacy system call entry.
*
* 32-bit x86 Linux system calls traditionally used the INT $0x80
* instruction. INT $0x80 lands here.
*
* This entry point can be used by any 32-bit perform system calls.
* Instances of INT $0x80 can be found inline in various programs and
* libraries. It is also used by the vDSO's __kernel_vsyscall
* fallback for hardware that doesn't support a faster entry method.
* Restarted 32-bit system calls also fall back to INT $0x80
* regardless of what instruction was originally used to do the system
* call. (64-bit programs can use INT $0x80 as well, but they can
* only run on 64-bit kernels and therefore land in
* entry_INT80_compat.)
*
* This is considered a slow path. It is not used by most libc
* implementations on modern hardware except during process startup.
*
* Arguments:
* eax system call number
* ebx arg1
* ecx arg2
* edx arg3
* esi arg4
* edi arg5
* ebp arg6
*/
ENTRY(entry_INT80_32)
ASM_CLAC
pushl %eax /* pt_regs->orig_ax */