linux-kernel - Re: Proposal for finishing the 64-bit x86 syscall cleanup

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Message-ID: <20150825084201.GA21589@gmail.com>
Date:	Tue, 25 Aug 2015 10:42:01 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	X86 ML <x86@...nel.org>, Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jan Beulich <jbeulich@...e.com>
Subject: Re: Proposal for finishing the 64-bit x86 syscall cleanup


* Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Andy Lutomirski <luto@...capital.net> wrote:
> 
> > Hi all-
> > 
> > I want to (try to) mostly or fully get rid of the messy bits (as
> > opposed to the hardware-bs-forced bits) of the 64-bit syscall asm.
> > There are two major conceptual things that are in the way.
> > 
> > Thing 1: partial pt_regs
> > 
> > 64-bit fast path syscalls don't fully initialize pt_regs: bx, bp, and
> > r12-r15 are uninitialized.  Some syscalls require them to be
> > initialized, and they have special awful stubs to do it.  The entry
> > and exit tracing code (except for phase1 tracing) also need them
> > initialized, and they have their own messy initialization.  Compat
> > syscalls are their own private little mess here.
> > 
> > This gets in the way of all kinds of cleanups, because C code can't
> > switch between the full and partial pt_regs states.
> > 
> > I can see two ways out.  We could remove the optimization entirely,
> > which consists of pushing and popping six more registers and adds
> > about ten cycles to fast path syscalls on Sandy Bridge.  It also
> > simplifies and presumably speeds up the slow paths.
> 
> So out of hundreds of regular system calls there's only a handful of such system 
> calls:
> 
> triton:~/tip> git grep stub arch/x86/entry/syscalls/
> arch/x86/entry/syscalls/syscall_32.tbl:2        i386    fork                    sys_fork                        stub32_fork
> arch/x86/entry/syscalls/syscall_32.tbl:11       i386    execve                  sys_execve                      stub32_execve
> arch/x86/entry/syscalls/syscall_32.tbl:119      i386    sigreturn               sys_sigreturn                   stub32_sigreturn
> arch/x86/entry/syscalls/syscall_32.tbl:120      i386    clone                   sys_clone                       stub32_clone
> arch/x86/entry/syscalls/syscall_32.tbl:173      i386    rt_sigreturn            sys_rt_sigreturn                stub32_rt_sigreturn
> arch/x86/entry/syscalls/syscall_32.tbl:190      i386    vfork                   sys_vfork                       stub32_vfork
> arch/x86/entry/syscalls/syscall_32.tbl:358      i386    execveat                sys_execveat                    stub32_execveat
> arch/x86/entry/syscalls/syscall_64.tbl:15       64      rt_sigreturn            stub_rt_sigreturn
> arch/x86/entry/syscalls/syscall_64.tbl:56       common  clone                   stub_clone
> arch/x86/entry/syscalls/syscall_64.tbl:57       common  fork                    stub_fork
> arch/x86/entry/syscalls/syscall_64.tbl:58       common  vfork                   stub_vfork
> arch/x86/entry/syscalls/syscall_64.tbl:59       64      execve                  stub_execve
> arch/x86/entry/syscalls/syscall_64.tbl:322      64      execveat                stub_execveat
> arch/x86/entry/syscalls/syscall_64.tbl:513      x32     rt_sigreturn            stub_x32_rt_sigreturn
> arch/x86/entry/syscalls/syscall_64.tbl:520      x32     execve                  stub_x32_execve
> arch/x86/entry/syscalls/syscall_64.tbl:545      x32     execveat                stub_x32_execveat
> 
> and none of them are super performance critical system calls, so no way would I 
> go for unconditionally saving/restoring all of ptregs, just to make it a bit 
> simpler for these syscalls.

Let me qualify that: no way in the long run.

In the short run we can drop the optimization and reintroduce it later, to lower 
all the risks that the C conversion brings with itself.

( That would also make it easier to re-analyze the cost/benefit ratio of the 
  optimization. )

So feel free to introduce a simple ptregs save/restore pattern for now.

Thanks,

	Ingo
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