* Denys Vlasenko <[email protected]> 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 <[email protected]>
> CC: Linus Torvalds <[email protected]>
> CC: Steven Rostedt <[email protected]>
> CC: Ingo Molnar <[email protected]>
> CC: Borislav Petkov <[email protected]>
> CC: "H. Peter Anvin" <[email protected]>
> CC: Andy Lutomirski <[email protected]>
> CC: Oleg Nesterov <[email protected]>
> CC: Frederic Weisbecker <[email protected]>
> CC: Alexei Starovoitov <[email protected]>
> CC: Will Drewry <[email protected]>
> CC: Kees Cook <[email protected]>
> CC: [email protected]
> CC: [email protected]
> ---
>
> 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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