Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752836AbbLIVI0 (ORCPT ); Wed, 9 Dec 2015 16:08:26 -0500 Received: from mail-vk0-f44.google.com ([209.85.213.44]:35710 "EHLO mail-vk0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752187AbbLIVIX convert rfc822-to-8bit (ORCPT ); Wed, 9 Dec 2015 16:08:23 -0500 MIME-Version: 1.0 In-Reply-To: References: <1449666173-15366-1-git-send-email-brgerst@gmail.com> Date: Wed, 9 Dec 2015 16:08:22 -0500 Message-ID: Subject: Re: [PATCH] x86/entry/64: Remove duplicate syscall table for fast path From: Brian Gerst To: Andy Lutomirski Cc: Andy Lutomirski , "the arch/x86 maintainers" , Linux Kernel Mailing List , Borislav Petkov , =?UTF-8?B?RnLDqWTDqXJpYyBXZWlzYmVja2Vy?= , Denys Vlasenko , Linus Torvalds Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2996 Lines: 90 On Wed, Dec 9, 2015 at 1:53 PM, Andy Lutomirski wrote: > On Wed, Dec 9, 2015 at 5:02 AM, Brian Gerst wrote: >> Instead of using a duplicate syscall table for the fast path, create stubs for >> the syscalls that need pt_regs that save the extra registers if a flag for the >> slow path is not set. >> >> Signed-off-by: Brian Gerst >> To: Andy Lutomirski >> Cc: Andy Lutomirski >> Cc: the arch/x86 maintainers >> Cc: Linux Kernel Mailing List >> Cc: Borislav Petkov >> Cc: Frédéric Weisbecker >> Cc: Denys Vlasenko >> Cc: Linus Torvalds >> --- >> >> Applies on top of Andy's syscall cleanup series. > > A couple questions: > >> @@ -306,15 +306,37 @@ END(entry_SYSCALL_64) >> >> ENTRY(stub_ptregs_64) >> /* >> - * Syscalls marked as needing ptregs that go through the fast path >> - * land here. We transfer to the slow path. >> + * Syscalls marked as needing ptregs land here. >> + * If we are on the fast path, we need to save the extra regs. >> + * If we are on the slow path, the extra regs are already saved. >> */ >> - DISABLE_INTERRUPTS(CLBR_NONE) >> - TRACE_IRQS_OFF >> - addq $8, %rsp >> - jmp entry_SYSCALL64_slow_path >> + movq PER_CPU_VAR(cpu_current_top_of_stack), %r10 >> + testl $TS_SLOWPATH, ASM_THREAD_INFO(TI_status, %r10, 0) >> + jnz 1f > > OK (but see below), but why not do: > > addq $8, %rsp > jmp entry_SYSCALL64_slow_path I've always been adverse to doing things like that because it breaks call/return branch prediction. Also, are there any side effects to calling enter_from_user_mode() more than once? > here instead of the stack munging below? > >> + subq $SIZEOF_PTREGS, %r10 >> + SAVE_EXTRA_REGS base=r10 >> + movq %r10, %rbx >> + call *%rax >> + movq %rbx, %r10 >> + RESTORE_EXTRA_REGS base=r10 >> + ret >> +1: >> + jmp *%rax >> END(stub_ptregs_64) After some thought, that can be simplified. It's only executed on the fast path, so pt_regs is at 8(%rsp). > Also, can we not get away with keying off rip or rsp instead of > ti->status? That should be faster and less magical IMO. Checking if the return address is the instruction after the fast path dispatch would work. Simplified version: ENTRY(stub_ptregs_64) cmpl $fast_path_return, (%rsp) jne 1f SAVE_EXTRA_REGS offset=8 call *%rax RESTORE_EXTRA_REGS offset=8 ret 1: jmp *%rax END(stub_ptregs_64) -- Brian Gerst -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/