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Subject: Re: [PATCH v3 0/6] Static calls
From:   "H. Peter Anvin" <hpa@zytor.com>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Jiri Kosina <jikos@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Nadav Amit <namit@vmware.com>,
        Peter Zijlstra <peterz@infradead.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jason Baron <jbaron@akamai.com>,
        David Laight <David.Laight@aculab.com>,
        Borislav Petkov <bp@alien8.de>,
        Julia Cartwright <julia@ni.com>, Jessica Yu <jeyu@kernel.org>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Edward Cree <ecree@solarflare.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>
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Message-ID: <bd8e2753-e460-7641-5b52-e4cb9ab1fc10@zytor.com>
Date:   Mon, 14 Jan 2019 21:37:44 -0800
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On 1/14/19 9:01 PM, H. Peter Anvin wrote:
> 
> This could be as simple as spinning for a limited time waiting for
> states 0 or 3 if we are not the patching CPU. It is also not necessary
> to wait for the mask to become zero for the first sync if we find
> ourselves suddenly in state 4.
> 

So this would look something like this for the #BP handler; I think this
is safe.  This uses the TLB miss on the write page intentionally to slow
down the loop a bit to reduce the risk of livelock.  Note that
"bp_write_addr" here refers to the write address for the breakpoint that
was taken.

	state = atomic_read(&bp_poke_state);
	if (state == 0)
		return 0;		/* No patching in progress */

recheck:
	clear bit in mask

	switch (state) {
		case 1:
		case 4:
			if (smp_processor_id() != bp_patching_cpu) {
				int retries = NNN;
				while (retries--) {
					invlpg
					if (*bp_write_addr != 0xcc)
						goto recheck;
					state = atomic_read(&bp_poke_state);
					if (state != 1 && state != 4)
						goto recheck;
				}
			}
			state = cmpxchg(&bp_poke_state, 1, 4);
			if (state != 1 && state != 4)
				goto recheck;
			atomic_write(bp_write_addr, bp_old_value);
			break;
		case 2:
			if (smp_processor_id() != bp_patching_cpu) {
				invlpg
				state = atomic_read(&bp_poke_state);
				if (state != 2)
					goto recheck;
			}
			complete patch sequence
			remove breakpoint
			break;

		case 3:
		case 0:
			/*
			 * If we are here, the #BP will go away on its
			 * own, or we will re-take it if it was a "real"
			 * breakpoint.
			 */
			break;
	}
	return 1;