Messages in this thread | | | Date | Wed, 25 Mar 2015 19:38:42 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 2/2] x86/asm/entry/64: do not TRACE_IRQS fast SYSRET64 path |
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* 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
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