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SubjectRe: [RFC PATCH 6/6] x86/jump_label,x86/alternatives: Batch jump label transformations
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.

-- Steve


> + return 1;
> + }
> + }
>
> + return 0;
> }
>
> static void text_poke_bp_set_handler(void *addr, void *handler,
> unsigned char int3)
> {
> - bp_int3_handler = handler;
> - bp_int3_addr = (u8 *)addr + sizeof(int3);
> text_poke(addr, &int3, sizeof(int3));
> }
>
> @@ -812,6 +832,9 @@ void *text_poke_bp(void *addr, const void *opcode, size_t len, void *handler)
>
> lockdep_assert_held(&text_mutex);
>
> + bp_int3_handler = handler;
> + bp_int3_addr = (u8 *)addr + sizeof(int3);
> +
> bp_patching_in_progress = true;
> /*
> * Corresponding read barrier in int3 notifier for making sure the
> @@ -841,7 +864,53 @@ void *text_poke_bp(void *addr, const void *opcode, size_t len, void *handler)
> * the writing of the new instruction.
> */
> bp_patching_in_progress = false;
> -
> + bp_int3_handler = bp_int3_addr = 0;
> return addr;
> }
>
> +void text_poke_bp_list(struct list_head *entry_list)
> +{
> + unsigned char int3 = 0xcc;
> + int patched_all_but_first = 0;
> + struct text_to_poke *tp;
> +
> + bp_list = entry_list;
> + bp_patching_in_progress = true;
> + /*
> + * Corresponding read barrier in int3 notifier for making sure the
> + * in_progress and handler are correctly ordered wrt. patching.
> + */
> + smp_wmb();
> +
> + list_for_each_entry(tp, entry_list, list)
> + text_poke_bp_set_handler(tp->addr, tp->handler, int3);
> +
> + on_each_cpu(do_sync_core, NULL, 1);
> +
> + list_for_each_entry(tp, entry_list, list) {
> + if (tp->len - sizeof(int3) > 0) {
> + patch_all_but_first_byte(tp->addr, tp->opcode, tp->len, int3);
> + patched_all_but_first++;
> + }
> + }
> +
> + if (patched_all_but_first) {
> + /*
> + * According to Intel, this core syncing is very likely
> + * not necessary and we'd be safe even without it. But
> + * better safe than sorry (plus there's not only Intel).
> + */
> + on_each_cpu(do_sync_core, NULL, 1);
> + }
> +
> + list_for_each_entry(tp, entry_list, list)
> + patch_first_byte(tp->addr, tp->opcode, int3);
> +
> + on_each_cpu(do_sync_core, NULL, 1);
> + /*
> + * sync_core() implies an smp_mb() and orders this store against
> + * the writing of the new instruction.
> + */
> + bp_list = 0;
> + bp_patching_in_progress = false;
> +}

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