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SubjectRe: [PATCH v3 2/7] x86/jump_label: Use text_poke_early() during early_init
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> On Nov 5, 2018, at 6:09 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>
>> On Fri, Nov 02, 2018 at 04:29:41PM -0700, Nadav Amit wrote:
>> diff --git a/arch/x86/kernel/jump_label.c b/arch/x86/kernel/jump_label.c
>> index aac0c1f7e354..367c1d0c20a3 100644
>> --- a/arch/x86/kernel/jump_label.c
>> +++ b/arch/x86/kernel/jump_label.c
>> @@ -52,7 +52,13 @@ static void __ref __jump_label_transform(struct jump_entry *entry,
>> jmp.offset = jump_entry_target(entry) -
>> (jump_entry_code(entry) + JUMP_LABEL_NOP_SIZE);
>>
>> - if (early_boot_irqs_disabled)
>> + /*
>> + * As long as we are in early boot, we can use text_poke_early(), which
>> + * is more efficient: the memory was still not marked as read-only (it
>> + * is only marked after poking_init()). This also prevents us from using
>> + * text_poke() before poking_init() is called.
>> + */
>> + if (!early_boot_done)
>> poker = text_poke_early;
>>
>> if (type == JUMP_LABEL_JMP) {
>
> It took me a while to untangle init/maze^H^Hin.c... but I think this
> is all we need:
>
> diff --git a/arch/x86/kernel/jump_label.c b/arch/x86/kernel/jump_label.c
> index aac0c1f7e354..ed5fe274a7d8 100644
> --- a/arch/x86/kernel/jump_label.c
> +++ b/arch/x86/kernel/jump_label.c
> @@ -52,7 +52,12 @@ static void __ref __jump_label_transform(struct jump_entry *entry,
> jmp.offset = jump_entry_target(entry) -
> (jump_entry_code(entry) + JUMP_LABEL_NOP_SIZE);
>
> - if (early_boot_irqs_disabled)
> + /*
> + * As long as we're UP and not yet marked RO, we can use
> + * text_poke_early; SYSTEM_BOOTING guarantees both, as we switch to
> + * SYSTEM_SCHEDULING before going either.
> + */
> + if (system_state == SYSTEM_BOOTING)
> poker = text_poke_early;
>
> if (type == JUMP_LABEL_JMP) {

Can we move this logic into text_poke() and get rid of text_poke_early()?

FWIW, alternative patching was, at some point, a significant fraction of total boot time in some cases. This was probably mostly due to unnecessary sync_core() calls. Although I think this was reported on a VM, and sync_core() used to be *extremely* expensive on a VM, but that’s fixed now, and it even got backported, I think.

(Hmm. Maybe we can also make jump label patching work in early boot, too!)
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