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Date:   Tue, 23 Jan 2018 11:44:20 +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>,
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        Tim Chen <tim.c.chen@linux.intel.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        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 11:15 +0100, Ingo Molnar wrote:
> > 
> > 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.
> 
> Hm? We still have GCC emitting 'call __fentry__' don't we? Would be nice to get 
> to the point where we can patch *that* out into a NOP... or are you saying we 
> already can?

Yes, we already can and do patch the 'call __fentry__/ mcount' call site into a 
NOP today - all 50,000+ call sites on a typical distro kernel.

We did so for a long time - this is all a well established, working mechanism.

> But this is a digression. I was being pedantic about the "0 cycles" but sure, 
> this would be perfectly tolerable.

It's not a digression in two ways:

- I wanted to make it clear that for distro kernels it _is_ a zero cycles overhead
  mechanism for non-SkyLake CPUs, literally.

- I noticed that Meltdown and the CR3 writes for PTI appears to have established a
  kind of ... insensitivity and numbness to kernel micro-costs, which peaked with
  the per-syscall MSR write nonsense patch of the SkyLake workaround.
  That attitude is totally unacceptable to me as x86 maintainer and yes, still
  every cycle counts.

Thanks,

	Ingo