Re: [PATCH 2/2] x86/asm/entry/64: do not TRACE_IRQS fast SYSRET64 path

[Ingo Molnar <mingo@kernel.org>]

* Denys Vlasenko <dvlasenk@redhat.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@redhat.com>
> CC: Linus Torvalds <torvalds@linux-foundation.org>
> CC: Steven Rostedt <rostedt@goodmis.org>
> CC: Ingo Molnar <mingo@kernel.org>
> CC: Borislav Petkov <bp@alien8.de>
> CC: "H. Peter Anvin" <hpa@zytor.com>
> CC: Andy Lutomirski <luto@amacapital.net>
> CC: Oleg Nesterov <oleg@redhat.com>
> CC: Frederic Weisbecker <fweisbec@gmail.com>
> CC: Alexei Starovoitov <ast@plumgrid.com>
> CC: Will Drewry <wad@chromium.org>
> CC: Kees Cook <keescook@chromium.org>
> CC: x86@kernel.org
> CC: linux-kernel@vger.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