> On Oct 17, 2018, at 5:54 PM, Nadav Amit <na...@vmware.com> wrote: > > It is sometimes beneficial to prevent preemption for very few > instructions, or prevent preemption for some instructions that precede > a branch (this latter case will be introduced in the next patches). > > To provide such functionality on x86-64, we use an empty REX-prefix > (opcode 0x40) as an indication that preemption is disabled for the > following instruction.
Nifty! That being said, I think you have a few bugs. First, you can’t just ignore a rescheduling interrupt, as you introduce unbounded latency when this happens — you’re effectively emulating preempt_enable_no_resched(), which is not a drop-in replacement for preempt_enable(). To fix this, you may need to jump to a slow-path trampoline that calls schedule() at the end or consider rewinding one instruction instead. Or use TF, which is only a little bit terrifying... You also aren’t accounting for the case where you get an exception that is, in turn, preempted. > > It is expected that this opcode is not in common use. > > Signed-off-by: Nadav Amit <na...@vmware.com> > --- > arch/x86/entry/entry_64.S | 10 ++++++++++ > arch/x86/include/asm/nospec-branch.h | 12 ++++++++++++ > 2 files changed, 22 insertions(+) > > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S > index cb8a5893fd33..31d59aad496e 100644 > --- a/arch/x86/entry/entry_64.S > +++ b/arch/x86/entry/entry_64.S > @@ -643,6 +643,16 @@ retint_kernel: > jnc 1f > 0: cmpl $0, PER_CPU_VAR(__preempt_count) > jnz 1f > + > + /* > + * Allow to use hint to prevent preemption on a certain instruction. > + * Consider an instruction with the first byte having REX prefix > + * without any bits set as an indication for preemption disabled. > + */ > + movq RIP(%rsp), %rax > + cmpb $PREEMPT_DISABLE_PREFIX, (%rax) > + jz 1f > + > call preempt_schedule_irq > jmp 0b > 1: > diff --git a/arch/x86/include/asm/nospec-branch.h > b/arch/x86/include/asm/nospec-branch.h > index 80dc14422495..0267611eb247 100644 > --- a/arch/x86/include/asm/nospec-branch.h > +++ b/arch/x86/include/asm/nospec-branch.h > @@ -52,6 +52,12 @@ > jnz 771b; \ > add $(BITS_PER_LONG/8) * nr, sp; > > +/* > + * An empty REX-prefix is an indication that preemption should not take > place on > + * this instruction. > + */ > +#define PREEMPT_DISABLE_PREFIX (0x40) > + > #ifdef __ASSEMBLY__ > > /* > @@ -148,6 +154,12 @@ > #endif > .endm > > +.macro preempt_disable_prefix > +#ifdef CONFIG_PREEMPT > + .byte PREEMPT_DISABLE_PREFIX > +#endif > +.endm > + > #else /* __ASSEMBLY__ */ > > #define ANNOTATE_NOSPEC_ALTERNATIVE \ > -- > 2.17.1 >
