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SubjectRe: [RFC PATCH 6/6] x86/jump_label,x86/alternatives: Batch jump label transformations
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On 10/10/18 10:17 PM, Steven Rostedt wrote:
> On Mon, 8 Oct 2018 14:53:05 +0200
> Daniel Bristot de Oliveira <bristot@redhat.com> wrote:
>
>> diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
>> index 8c0de4282659..d61c476046fe 100644
>> --- a/arch/x86/include/asm/jump_label.h
>> +++ b/arch/x86/include/asm/jump_label.h
>> @@ -15,6 +15,8 @@
>> #error asm/jump_label.h included on a non-jump-label kernel
>> #endif
>>
>> +#define HAVE_JUMP_LABEL_BATCH
>> +
>> #define JUMP_LABEL_NOP_SIZE 5
>>
>> #ifdef CONFIG_X86_64
>> diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
>> index e85ff65c43c3..a28230f09d72 100644
>> --- a/arch/x86/include/asm/text-patching.h
>> +++ b/arch/x86/include/asm/text-patching.h
>> @@ -18,6 +18,14 @@ static inline void apply_paravirt(struct paravirt_patch_site *start,
>> #define __parainstructions_end NULL
>> #endif
>>
>> +struct text_to_poke {
>> + struct list_head list;
>> + void *opcode;
>> + void *addr;
>> + void *handler;
>> + size_t len;
>> +};
>> +
>> extern void *text_poke_early(void *addr, const void *opcode, size_t len);
>>
>> /*
>> @@ -37,6 +45,7 @@ extern void *text_poke_early(void *addr, const void *opcode, size_t len);
>> extern void *text_poke(void *addr, const void *opcode, size_t len);
>> extern int poke_int3_handler(struct pt_regs *regs);
>> extern void *text_poke_bp(void *addr, const void *opcode, size_t len, void *handler);
>> +extern void text_poke_bp_list(struct list_head *entry_list);
>> extern int after_bootmem;
>>
>> #endif /* _ASM_X86_TEXT_PATCHING_H */
>> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
>> index a4c83cb49cd0..3bd502ea4c53 100644
>> --- a/arch/x86/kernel/alternative.c
>> +++ b/arch/x86/kernel/alternative.c
>> @@ -735,9 +735,12 @@ static void do_sync_core(void *info)
>>
>> static bool bp_patching_in_progress;
>> static void *bp_int3_handler, *bp_int3_addr;
>> +struct list_head *bp_list;
>>
>> int poke_int3_handler(struct pt_regs *regs)
>> {
>> + void *ip;
>> + struct text_to_poke *tp;
>> /*
>> * Having observed our INT3 instruction, we now must observe
>> * bp_patching_in_progress.
>> @@ -753,21 +756,38 @@ int poke_int3_handler(struct pt_regs *regs)
>> if (likely(!bp_patching_in_progress))
>> return 0;
>>
>> - if (user_mode(regs) || regs->ip != (unsigned long)bp_int3_addr)
>> + if (user_mode(regs))
>> return 0;
>>
>> - /* set up the specified breakpoint handler */
>> - regs->ip = (unsigned long) bp_int3_handler;
>> + /*
>> + * Single poke.
>> + */
>> + if (bp_int3_addr) {
>> + if (regs->ip == (unsigned long) bp_int3_addr) {
>> + regs->ip = (unsigned long) bp_int3_handler;
>> + return 1;
>> + }
>> + return 0;
>> + }
>>
>> - return 1;
>> + /*
>> + * Batch mode.
>> + */
>> + ip = (void *) regs->ip - sizeof(unsigned char);
>> + list_for_each_entry(tp, bp_list, list) {
>> + if (ip == tp->addr) {
>> + /* set up the specified breakpoint handler */
>> + regs->ip = (unsigned long) tp->handler;
>
> I hate this loop. Makes hitting the static branch (which is probably in
> a fast path, otherwise why use static branches?), do a O(n) loop of
> batch updates. You may not have that many, but why not optimize it?
>
> Can we make an array of the handlers, in sorted order, then we simply
> do a binary search for the ip involved? Turning this from O(n) to
> O(log2(n)).
>
> As Jason mentioned. If you set aside a page for processing batches up
> to PAGE / (sizeof tp) then you can also make it sorted and replace the
> iteration with a loop.

Hi Steven!

I am convinced! I am working in the v2 now, that will:

Split the batch mode into two patches (Jason)
Use an aside page rather than allocating memory (Jason)
Use a sorted vector of keys with binary search in the int3 handler (Steven)

I will also do some performance tests in the int3 handler (peterz on IRC).

Thanks!

-- Daniel

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