On Mon, Sep 23, 2019 at 01:49:20PM +0200, Peter Zijlstra wrote: > While walking the kids to school I wondered WTH we need to call > TRACE_IRQS_OFF in the first place. If this is the return from exception > path, interrupts had better be disabled already (in exception enter). > > For entry_64.S we have: > > - idtentry_part; which does TRACE_IRQS_OFF at the start and error_exit > at the end. > > - xen_do_hypervisor_callback, xen_failsafe_callback -- which are > confusing. > > So in the normal case, it appears we can simply do: > > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S > index b7c3ea4cb19d..e9cf59ac554e 100644 > --- a/arch/x86/entry/entry_64.S > +++ b/arch/x86/entry/entry_64.S > @@ -1368,8 +1368,6 @@ END(error_entry) > > ENTRY(error_exit) > UNWIND_HINT_REGS > - DISABLE_INTERRUPTS(CLBR_ANY) > - TRACE_IRQS_OFF > testb $3, CS(%rsp) > jz retint_kernel > jmp retint_user > > and all should be well. This leaves Xen... > > For entry_32.S it looks like nothing uses 'resume_userspace' so that > ENTRY can go away. Which then leaves: > > * ret_from_intr: > - common_spurious: TRACE_IRQS_OFF > - common_interrupt: TRACE_IRQS_OFF > - BUILD_INTERRUPT3: TRACE_IRQS_OFF > - xen_do_upcall: ... > > * ret_from_exception: > - xen_failsafe_callback: ... > - common_exception_read_cr2: TRACE_IRQS_OFF > - common_exception: TRACE_IRQS_OFF > - int3: TRACE_IRQS_OFF > > Which again suggests: > > diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S > index f83ca5aa8b77..7a19e7413a8e 100644 > --- a/arch/x86/entry/entry_32.S > +++ b/arch/x86/entry/entry_32.S > @@ -825,9 +825,6 @@ END(ret_from_fork) > cmpl $USER_RPL, %eax > jb restore_all_kernel # not returning to v8086 or userspace > > -ENTRY(resume_userspace) > - DISABLE_INTERRUPTS(CLBR_ANY) > - TRACE_IRQS_OFF > movl %esp, %eax > call prepare_exit_to_usermode > jmp restore_all > > with the notable exception (oh teh pun!) being Xen... _again_. > > With these patchlets on, we'd want prepare_exit_to_usermode() to > validate the IRQ state, but AFAICT it _should_ all just 'work' (famous > last words). > > Cc Juergen because Xen Arrgh.. faults!! they do local_irq_enable() but never disable them again. Arguably those functions should be symmetric and restore IF when they muck with it instead of relying on the exit path fixing things up.
