linux-kernel - Re: [PATCH 2/2] x86/asm/entry/64: do not TRACE

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Date:	Wed, 25 Mar 2015 19:38:42 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] x86/asm/entry/64: do not TRACE_IRQS fast SYSRET64
 path


* Denys Vlasenko <dvlasenk@...hat.com> wrote:

> SYSRET code path has a small irq-off block.
> On this code path, TRACE_IRQS_ON can't be called right before interrupts
> are enabled for real, we can't clobber registers there.
> So current code does it earlier, in a safe place.
> 
> But with this, TRACE_IRQS_OFF/ON frames just two fast instructions,
> which is ridiculous: now most of irq-off block is _outside_ of the framing.
> 
> Do the same thing that we do on SYSCALL entry: do not track this irq-off block,
> it is very small to ever cause noticeable irq latency.
> 
> Be careful: make sure that "jnz int_ret_from_sys_call_irqs_off" now does
> invoke TRACE_IRQS_OFF - move int_ret_from_sys_call_irqs_off label before
> TRACE_IRQS_OFF.
> 
> Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
> CC: Linus Torvalds <torvalds@...ux-foundation.org>
> CC: Steven Rostedt <rostedt@...dmis.org>
> CC: Ingo Molnar <mingo@...nel.org>
> CC: Borislav Petkov <bp@...en8.de>
> CC: "H. Peter Anvin" <hpa@...or.com>
> CC: Andy Lutomirski <luto@...capital.net>
> CC: Oleg Nesterov <oleg@...hat.com>
> CC: Frederic Weisbecker <fweisbec@...il.com>
> CC: Alexei Starovoitov <ast@...mgrid.com>
> CC: Will Drewry <wad@...omium.org>
> CC: Kees Cook <keescook@...omium.org>
> CC: x86@...nel.org
> CC: linux-kernel@...r.kernel.org
> ---
> 
> Changes in v2: added comment
> 
>  arch/x86/kernel/entry_64.S | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index 9c8661c..658cf2e 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -269,8 +269,11 @@ system_call_fastpath:
>   * Has incompletely filled pt_regs.
>   */
>  	LOCKDEP_SYS_EXIT
> +	/*
> +	 * We do not frame this tiny irq-off block with TRACE_IRQS_OFF/ON,
> +	 * it is too small to ever cause noticeable irq latency.

         * ... but if we enter the slowpath from here, we'll execute a 
         * proper TRACE_IRQS_OFF call.

> @@ -298,6 +298,7 @@ system_call_fastpath:
>  	 * 64bit SYSRET restores rip from rcx,
>  	 * rflags from r11 (but RF and VM bits are forced to 0),
>  	 * cs and ss are loaded from MSRs.
> +	 * Restoration of rflags re-enables interrupts.
>  	 */
>  	USERGS_SYSRET64

Is that true even if user-space disabled irqs (via CLI) and executed a 
syscall while having irqs off?

Thanks,

	Ingo
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