linux-kernel - RE: [PATCH 0/8] arm64/ftrace: Add support for DYNAMIC_FTRACE_WITH_CALL

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Date:   Tue, 10 Jan 2023 08:55:58 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Mark Rutland' <mark.rutland@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
CC:     "catalin.marinas@....com" <catalin.marinas@....com>,
        "lenb@...nel.org" <lenb@...nel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mhiramat@...nel.org" <mhiramat@...nel.org>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
        "ojeda@...nel.org" <ojeda@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "rafael.j.wysocki@...el.com" <rafael.j.wysocki@...el.com>,
        "revest@...omium.org" <revest@...omium.org>,
        "robert.moore@...el.com" <robert.moore@...el.com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "will@...nel.org" <will@...nel.org>
Subject: RE: [PATCH 0/8] arm64/ftrace: Add support for
 DYNAMIC_FTRACE_WITH_CALL_OPS

From: Mark Rutland
> Sent: 09 January 2023 13:58
> 
> This series adds a new DYNAMIC_FTRACE_WITH_CALL_OPS mechanism, and
> enables support for this on arm64. This significantly reduces the
> overhead of tracing when a callsite/tracee has a single associated
> tracer, avoids a number of issues that make it undesireably and
> infeasible to use dynamically-allocated trampolines (e.g. branch range
> limitations), and makes it possible to implement support for
> DYNAMIC_FTRACE_WITH_DIRECT_CALLS in future.
> 
> The main idea is to give each ftrace callsite an associated pointer to
> an ftrace_ops. The architecture's ftrace_caller trampoline can recover
> the ops pointer and invoke ops->func from this without needing to use
> ftrace_ops_list_func, which has to iterate through all registered ops.
> 
> To do this, we use -fpatchable-function-entry=M,N, there N NOPs are
> placed before the function entry point...

Doesn't this bump the minimum gcc version up to something like 9.0 ?

How does it interact with the 'CFI stuff' that also uses the same area?

	David

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