Re: svn commit: r220430 - head/sys/amd64/amd64

on 08/04/2011 00:32 John Baldwin said the following:
> 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

I think that this commit (plus r220431) has broken something in my environment.
After updating to the most recent head I started to get semi-random problems in
various areas:
- named would consistently fail to start, but with different errors (assertions)
- ^Z and fg result in a process getting SIGSEGV
- X sometimes fails to start complaining about failed VT switch

Reverting just these two commits restores sanity.

Just in case, my processor is AMD (arch is obviously amd64).

> 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)
> _at__at_ -339,6 +339,9 _at__at_ 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
> _at__at_ -380,6 +383,36 _at__at_ IDTVEC(fast_syscall)
>  	movq	%rsp,%rdi
>  	call	syscall
>  	movq	PCPU(CURPCB),%rax
> +	testq	$PCB_FULL_IRET,PCB_FLAGS(%rax)
> +	jne	3f
> +1:	/* Check for and handle AST's on return to userland. */
> +	cli
> +	movq	PCPU(CURTHREAD),%rax
> +	testl	$TDF_ASTPENDING | TDF_NEEDRESCHED,TD_FLAGS(%rax)
> +	je	2f
> +	sti
> +	movq	%rsp, %rdi
> +	call	ast
> +	jmp	1b
> +2:	/* Restore preserved registers. */
> +	MEXITCOUNT
> +	movq	TF_RDI(%rsp),%rdi	/* bonus; preserve arg 1 */
> +	movq	TF_RSI(%rsp),%rsi	/* bonus: preserve arg 2 */
> +	movq	TF_RDX(%rsp),%rdx	/* return value 2 */
> +	movq	TF_RAX(%rsp),%rax	/* return value 1 */
> +	movq	TF_RBX(%rsp),%rbx	/* C preserved */
> +	movq	TF_RBP(%rsp),%rbp	/* C preserved */
> +	movq	TF_R12(%rsp),%r12	/* C preserved */
> +	movq	TF_R13(%rsp),%r13	/* C preserved */
> +	movq	TF_R14(%rsp),%r14	/* C preserved */
> +	movq	TF_R15(%rsp),%r15	/* C preserved */
> +	movq	TF_RFLAGS(%rsp),%r11	/* original %rflags */
> +	movq	TF_RIP(%rsp),%rcx	/* original %rip */
> +	movq	TF_RSP(%rsp),%r9	/* user stack pointer */
> +	movq	%r9,%rsp		/* original %rsp */
> +	swapgs
> +	sysretq
> +3:	/* Requested full context restore, use doreti for that. */
>  	MEXITCOUNT
>  	jmp	doreti
>  


-- 
Andriy Gapon