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Date:   Fri, 1 Nov 2019 11:09:03 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Leo Yan <leo.yan@linaro.org>
Cc:     Will Deacon <will@kernel.org>, Mark Rutland <mark.rutland@arm.com>,
        Mike Leach <mike.leach@linaro.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Mathieu Poirier <mathieu.poirier@linaro.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Jiri Olsa <jolsa@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC 1/6] perf/x86: Add perf text poke event
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On Fri, Nov 01, 2019 at 11:04:40AM +0100, Peter Zijlstra wrote:

> I'm thinking something along the lines of:
> 
> static uintptr_t nosync_addr;
> static u32 nosync_insn;
> 
> int __kprobes aarch64_insn_patch_text_nosync(void *addr, u32 insn)
> {
> 	const u32 break = // some_breakpoint_insn;
> 	uintptr_t tp = (uintptr_t)addr;
> 	int ret;
> 
> 	lockdep_assert_held(&text_mutex);
> 
> 	/* A64 instructions must be word aligned */
> 	if (tp & 0x3)
> 		return -EINVAL;
> 
> 	if (perf_text_poke_update_enabled()) {
> 
> 		nosync_insn = insn;
> 		smp_store_release(&nosync_addr, tp);
> 
> 		ret = aarch64_insn_write(addr, break);
> 		if (ret == 0)
> 			__flush_icache_range(tp, tp + AARCH64_INSN_SIZE);
> 
> 		perf_event_text_poke(....);
> 	}
> 
> 	ret = aarch64_insn_write(addr, insn);
> 	if (ret == 0)
> 		__flush_icache_range(tp, tp + AARCH64_INSN_SIZE);

	// barrier that guarantees iflush completion ? dsb(osh) ?

	WRITE_ONCE(nosync_addr, NULL);

> 	return ret;
> }
> 
> And have the 'break' handler do:
> 
> aarch64_insn_break_handler(struct pt_regs *regs)
> {
> 	unsigned long addr = smp_load_acquire(&nosync_addr);
> 	u32 insn = nosync_insn;
> 
> 	if (regs->ip != addr)
> 		return;
> 
> 	// emulate @insn
> }
> 
> I understood from Will the whole nosync scheme only works for a limited
> set of instructions, but you only have to implement emulation for the
> actual instructions used of course.
> 
> (which is what we do on x86)
> 
> Does this sound workable?