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From: Jung-uk Kim <jkim@FreeBSD.org>
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Date: Thu, 7 Apr 2011 17:51:58 -0400
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Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org,
	John Baldwin <jhb@freebsd.org>
Subject: Re: svn commit: r220430 - head/sys/amd64/amd64
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On Thursday 07 April 2011 05:32 pm, John Baldwin wrote:
> Author: jhb
> Date: Thu Apr  7 21:32:25 2011
> New Revision: 220430
> URL: http://svn.freebsd.org/changeset/base/220430
>
> Log:
>   If a system call does not request a full interrupt return, use a
> fast path via the sysretq instruction to return from the system
> call.  This was removed in 190620 and not quite fully restored in
> 195486.  This resolves most of the performance regression in system
> call microbenchmarks between 7 and 8 on amd64.
>
>   Reviewed by:	kib
>   MFC after:	1 week
>
> Modified:
>   head/sys/amd64/amd64/exception.S
>
> Modified: head/sys/amd64/amd64/exception.S
> ===================================================================
>=========== --- head/sys/amd64/amd64/exception.S	Thu Apr  7 21:29:34
> 2011	(r220429) +++ head/sys/amd64/amd64/exception.S	Thu Apr  7
> 21:32:25 2011	(r220430) @@ -339,6 +339,9 @@ IDTVEC(prot)
>   * and the new privilige level.  We are still running on the old
> user stack * pointer.  We have to juggle a few things around to
> find our stack etc. * swapgs gives us access to our PCPU space
> only.
> + *
> + * We do not support invoking this from a custom %cs or %ss (e.g.
> using + * entries from an LDT).
>   */
>  IDTVEC(fast_syscall)
>  	swapgs
> @@ -380,6 +383,36 @@ IDTVEC(fast_syscall)
>  	movq	%rsp,%rdi
>  	call	syscall
>  	movq	PCPU(CURPCB),%rax
> +	testq	$PCB_FULL_IRET,PCB_FLAGS(%rax)

FYI, pcb_flags is u_int now, i.e., we need testl here.

Jung-uk Kim