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Re: [PATCH 00/10] x86: Various SYSENTER/SYSEXIT/#DB fixes and cleanups

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* Andy Lutomirski <luto@kernel.org> wrote:

> hpa asked me to get rid of the ASM_CLAC at the beginning of the SYSENTER
> path.  Little did he know...

Btw., before we further change this code, something else I think would be very 
useful. We have countless system call entry points on x86 CPUs, and they are now 
consistently named and are very easy to grep for:

 triton:~/tip> git grep 'ENTRY(entry_' arch/x86/entry/
 arch/x86/entry/entry_32.S:ENTRY(entry_SYSENTER_32)
 arch/x86/entry/entry_32.S:ENTRY(entry_INT80_32)
 arch/x86/entry/entry_64.S:ENTRY(entry_SYSCALL_64)
 arch/x86/entry/entry_64_compat.S:ENTRY(entry_SYSENTER_compat)
 arch/x86/entry/entry_64_compat.S:ENTRY(entry_SYSCALL_compat)
 arch/x86/entry/entry_64_compat.S:ENTRY(entry_INT80_compat)

Furthermore, each entry point has extensive comments, except one important detail: 
none of the comments really explains the circumstances under which the entry 
points are _used_ by user-space.

I'd like to see something like:

arch/x86/entry/entry_64.S:ENTRY(entry_SYSCALL_64)

	*
	* The 64-bit SYSCALL instruction is used by all modern 64-bit user-space 
	* code to execute most system calls: this instruction is the fastest and 
	* sanest implementation on modern Intel and AMD CPUs.
	*

... and we should add similar explanations for all of the 6 entry points, with 
caveats and limitations listed generously.

Especially valuable would be to list eventual 'strange' usages of the various 
syscall instructions, used by rare packages, compatibility layers, emulators, 
embedded libraries, etc. (To the extent we know about them, obviously.)

I.e. it would be very nice to do a full documentation of our current system call 
usage patterns, as utilized by user-space. Beyond the documentation value this 
will also help people prioritize optimizations between the various entry points - 
which should be optimized more, which entry point matters less, etc.

Thanks,

	Ingo

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[PATCH 00/10] x86: Various SYSENTER/SYSEXIT/#DB fixes and cleanups Andy Lutomirski <luto@kernel.org> - 2016-03-06 07:00 +0100
  [PATCH v2 06/10] x86/traps: Clear DR6 early in do_debug and improve the comment Andy Lutomirski <luto@kernel.org> - 2016-03-06 07:00 +0100
  [PATCH v2 01/10] selftests/x86: In syscall_nt, test NT|TF as well Andy Lutomirski <luto@kernel.org> - 2016-03-06 07:00 +0100
  [PATCH v2 05/10] x86/traps: Clear TIF_BLOCKSTEP on all debug exceptions Andy Lutomirski <luto@kernel.org> - 2016-03-06 07:00 +0100
  [PATCH v2 03/10] x86/entry/32: Filter NT and speed up AC filtering in SYSENTER Andy Lutomirski <luto@kernel.org> - 2016-03-06 07:00 +0100
  [PATCH v2 02/10] x86/entry/compat: In SYSENTER, sink AC clearing below the existing FLAGS test Andy Lutomirski <luto@kernel.org> - 2016-03-06 07:00 +0100
  Re: [PATCH 00/10] x86: Various SYSENTER/SYSEXIT/#DB fixes and  cleanups Ingo Molnar <mingo@kernel.org> - 2016-03-06 09:30 +0100
    Re: [PATCH 00/10] x86: Various SYSENTER/SYSEXIT/#DB fixes and cleanups Andy Lutomirski <luto@amacapital.net> - 2016-03-06 17:20 +0100
      Re: [PATCH 00/10] x86: Various SYSENTER/SYSEXIT/#DB fixes and  cleanups Ingo Molnar <mingo@kernel.org> - 2016-03-07 09:30 +0100
  Re: [PATCH 00/10] x86: Various SYSENTER/SYSEXIT/#DB fixes and  cleanups Ingo Molnar <mingo@kernel.org> - 2016-03-06 09:40 +0100
    Re: [PATCH 00/10] x86: Various SYSENTER/SYSEXIT/#DB fixes and cleanups Andy Lutomirski <luto@amacapital.net> - 2016-03-06 17:20 +0100

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