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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat, 1 Sep 2018 14:38:43 -0700
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Subject: Re: Redoing eXclusive Page Frame Ownership (XPFO) with isolated CPUs
 in mind (for KVM to isolate its guests per CPU)
To:     jsteckli@amazon.de
Cc:     David Woodhouse <dwmw@amazon.co.uk>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
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        Jim Mattson <jmattson@google.com>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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On Fri, Aug 31, 2018 at 12:45 AM Julian Stecklina <jsteckli@amazon.de> wrote:
>
> I've been spending some cycles on the XPFO patch set this week. For the
> patch set as it was posted for v4.13, the performance overhead of
> compiling a Linux kernel is ~40% on x86_64[1]. The overhead comes almost
> completely from TLB flushing. If we can live with stale TLB entries
> allowing temporary access (which I think is reasonable), we can remove
> all TLB flushing (on x86). This reduces the overhead to 2-3% for
> kernel compile.

I have to say, even 2-3% for a kernel compile sounds absolutely horrendous.

Kernel bullds are 90% user space at least for me, so a 2-3% slowdown
from a kernel is not some small unnoticeable thing.

           Linus