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Subject: Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict
 Indirect Branch Speculation
From:   David Woodhouse <dwmw2@infradead.org>
To:     Ingo Molnar <mingo@kernel.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>,
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        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>,
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        Masami Hiramatsu <mhiramat@kernel.org>,
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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>
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Date:   Tue, 23 Jan 2018 10:27:24 +0000
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On Tue, 2018-01-23 at 11:15 +0100, Ingo Molnar wrote:
>=20
> BTW., the reason this is enabled on all distro kernels is because the ove=
rhead is=C2=A0
> a single patched-in NOP instruction in the function epilogue, when tracin=
g is=C2=A0
> 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?

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