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Date:   Tue, 23 Jan 2018 11:15:32 +0100
From:   Ingo Molnar <mingo@kernel.org>
To:     David Woodhouse <dwmw2@infradead.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        KarimAllah Ahmed <karahmed@amazon.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andi Kleen <ak@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Asit Mallick <asit.k.mallick@intel.com>,
        Borislav Petkov <bp@suse.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "H . Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@amd.com>,
        Joerg Roedel <joro@8bytes.org>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Laura Abbott <labbott@redhat.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tim Chen <tim.c.chen@linux.intel.com>,
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        KVM list <kvm@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Arjan Van De Ven <arjan.van.de.ven@intel.com>
Subject: Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict
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* David Woodhouse <dwmw2@infradead.org> wrote:

> On Tue, 2018-01-23 at 08:53 +0100, Ingo Molnar wrote:
> > 
> > The patch below demonstrates the principle, it forcibly enables dynamic ftrace?
> > patching (CONFIG_DYNAMIC_FTRACE=y et al) and turns mcount/__fentry__ into a RET:
> > 
> > ? ffffffff81a01a40 <__fentry__>:
> > ? ffffffff81a01a40:?????? c3????????????????????? retq???
> > 
> > This would have to be extended with (very simple) call stack depth tracking (just?
> > 3 more instructions would do in the fast path I believe) and a suitable SkyLake?
> > workaround (and also has to play nice with the ftrace callbacks).
> > 
> > On non-SkyLake the overhead would be 0 cycles.
> 
> The overhead of forcing CONFIG_DYNAMIC_FTRACE=y is precisely zero
> cycles? That seems a little optimistic. ;)

The overhead of the quick hack patch I sent to show what exact code I mean is 
obviously not zero.

The overhead of using my proposed solution, to utilize the function call callback 
that CONFIG_DYNAMIC_FTRACE=y provides, is exactly zero on non-SkyLake systems 
where the callback is patched out, on typical Linux distros.

The callback is widely enabled on distro kernels:

  Fedora:                    CONFIG_DYNAMIC_FTRACE=y
  Ubuntu:                    CONFIG_DYNAMIC_FTRACE=y
  OpenSuse (default flavor): CONFIG_DYNAMIC_FTRACE=y

BTW., the reason this is enabled on all distro kernels is because the overhead is 
a single patched-in NOP instruction in the function epilogue, when tracing is 
disabled. So it's not even a CALL+RET - it's a patched in NOP.

Thanks,

	Ingo