Messages in this thread Patches in this message | | | Date | Mon, 27 Jul 2020 15:05:36 +0200 | From | peterz@infradea ... | Subject | Re: [PATCH 4/4] x86/cpu: Use SERIALIZE in sync_core() when available |
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On Mon, Jul 27, 2020 at 05:49:28AM -0700, hpa@zytor.com wrote: > On July 27, 2020 1:36:19 AM PDT, peterz@infradead.org wrote: > >On Sun, Jul 26, 2020 at 10:55:15PM -0700, hpa@zytor.com wrote: > >> For a really overenginered solution, but which might perform > >> unnecessary poorly on existing hardware: > >> > >> asm volatile("1: .byte 0xf, 0x1, 0xe8; 2:" > >> _ASM_EXTABLE(1b,2b)); > > > >Ha! cute, you take an #UD ? > > > >We could optimize the #UD exception handler for this I suppose, but > >that makes it an even worse hack. The simple alternative() seems like > >a much simpler approach. > > If this is in any way performance critical, then no :)
Yeah, I'm not sure.. the 'funny' thing is that typically call sync_core() from an IPI anyway. And the synchronous broadcast IPI is by far the most expensive part of that.
Something like this...
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c index 20e07feb4064..528e049ee1d9 100644 --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -989,12 +989,13 @@ void *text_poke_kgdb(void *addr, const void *opcode, size_t len) static void do_sync_core(void *info) { - sync_core(); + /* IRET implies sync_core() */ } void text_poke_sync(void) { on_each_cpu(do_sync_core, NULL, 1); + sync_core(); } struct text_poke_loc {
> Taking the #UD > has the cute property that we end up IRET on the way back, so we don't > even need a fix-up path.
I got that, what I had in mind was making sure #UD avoids the overhead of doing exception entry/exit by adding an early exit.
Something like so:
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c index 8493f55e1167..a3f41d645944 100644 --- a/arch/x86/kernel/traps.c +++ b/arch/x86/kernel/traps.c @@ -96,6 +96,16 @@ __always_inline int is_valid_bugaddr(unsigned long addr) return *(unsigned short *)addr == INSN_UD2; } +__always_inline int handle_serialize(struct pt_regs *regs) +{ + const char serialize[3] = { 0x0f, 0xe8, 0x02 }; + + if (regs->ip < TASK_SIZE_MAX) + return 0; + + return !memcmp((const void *)regs->ip, serialize, 3); +} + static nokprobe_inline int do_trap_no_signal(struct task_struct *tsk, int trapnr, const char *str, struct pt_regs *regs, long error_code) @@ -252,8 +262,13 @@ DEFINE_IDTENTRY_RAW(exc_invalid_op) * handle it before exception entry to avoid recursive WARN * in case exception entry is the one triggering WARNs. */ - if (!user_mode(regs) && handle_bug(regs)) - return; + if (!user_mode(regs)) { + if (handle_bug(regs)) + return; + + if (handle_serialize(regs)) + return; + } state = idtentry_enter(regs); instrumentation_begin();
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