Messages in this thread | | | Date | Thu, 28 Jun 2012 14:08:49 +0200 | From | Frederic Weisbecker <> | Subject | Re: [RFC 09/23] x86_64: Store userspace rsp in system_call fastpath |
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On Tue, Jun 19, 2012 at 05:48:00PM +0200, Jiri Olsa wrote: > hi, > I'd need help with this change.. basically it works, but I guess > I could alter the FIXUP_TOP_OF_STACK macro as well, so the rsp is > not initialized twice in slowpath. > > But it seems quite complex.. so not really sure at the moment ;) > > ideas? > > thanks, > jirka > > --- > Storing the userspace rsp into the pt_regs struct for the > system_call fastpath (syscall instruction handler). > > Following part of the pt_regs is allocated on stack: > (via KERNEL_STACK_OFFSET) > > unsigned long ip; > unsigned long cs; > unsigned long flags; > unsigned long sp; > unsigned long ss; > > but only ip is actually saved for fastpath. > > For perf post unwind we need at least ip and sp to be able to > start the unwind, so storing the old_rsp value to the sp. > > Signed-off-by: Jiri Olsa <jolsa@redhat.com> > Cc: Borislav Petkov <bp@alien8.de> > Cc: H. Peter Anvin <hpa@zytor.com> > Cc: Roland McGrath <roland@hack.frob.com> > --- > arch/x86/kernel/entry_64.S | 5 +++++ > 1 files changed, 5 insertions(+), 0 deletions(-) > > diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S > index 111f6bb..0444917 100644 > --- a/arch/x86/kernel/entry_64.S > +++ b/arch/x86/kernel/entry_64.S > @@ -516,6 +516,11 @@ GLOBAL(system_call_after_swapgs) > SAVE_ARGS 8,0 > movq %rax,ORIG_RAX-ARGOFFSET(%rsp) > movq %rcx,RIP-ARGOFFSET(%rsp) > +#ifdef CONFIG_PERF_EVENTS > + /* We need rsp in fast path for perf post unwind. */ > + movq PER_CPU_VAR(old_rsp), %rcx > + movq %rcx,RSP-ARGOFFSET(%rsp) > +#endif
Another solution is to set/unset some TIF flag in perf_event_sched_in/out such that we take the syscall slow path (tracesys) which records every non-scratch registers. I can see that old_rsp is not saved in pt_regs by SAVE_REST so this may be something to add in tracesys.
This way we don't bloat the syscall fastpath with a feature only used by some developers.
> CFI_REL_OFFSET rip,RIP-ARGOFFSET > testl $_TIF_WORK_SYSCALL_ENTRY,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET) > jnz tracesys > -- > 1.7.7.6 >
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