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SubjectRe: [RFC PATCH 1/5] x86: introduce preemption disable prefix
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> On Oct 17, 2018, at 5:54 PM, Nadav Amit <namit@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 <namit@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
>

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