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Date:   Wed, 18 Nov 2020 12:29:52 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Alexandre Chartre <alexandre.chartre@oracle.com>
Cc:     tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com,
        x86@kernel.org, dave.hansen@linux.intel.com, luto@kernel.org,
        peterz@infradead.org, linux-kernel@vger.kernel.org,
        thomas.lendacky@amd.com, jroedel@suse.de, konrad.wilk@oracle.com,
        jan.setjeeilers@oracle.com, junaids@google.com, oweisse@google.com,
        rppt@linux.vnet.ibm.com, graf@amazon.de, mgross@linux.intel.com,
        kuzuno@gmail.com, Mel Gorman <mgorman@techsingularity.net>
Subject: Re: [RFC][PATCH v2 00/21] x86/pti: Defer CR3 switch to C code
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On Wed, Nov 18, 2020 at 08:41:42AM +0100, Alexandre Chartre wrote:
> Well, it looks like I wrongfully assume that KPTI was a well known performance
> overhead since it was introduced (because it adds extra page-table switches),
> but you are right I should be presenting my own numbers.

Here's one recipe, courtesy of Mel:

https://github.com/gormanm/mmtests

"
./run-mmtests.sh --no-monitor --config configs/config-workload-poundsyscall test-default

# reboot the machine with pti disabled

./run-mmtests.sh --no-monitor --config configs/config-workload-poundsyscall test-nopti

poundsyscall just calls getppid() so it's a light-weight syscall and a
proxy measure for syscall entry/exit costs. To do the actual compare

cd work/log
../../compare-kernels.sh

and see what gain there is from disabling pti. If you want to compare
the other direction

../../compare-kernels.sh --baseline test-nopti --compare test-default

If you get an error about BinarySearch

(echo y;echo o conf prerequisites_policy follow;echo o conf commit)|cpan
yes | cpan List::BinarySearch

Only se the second line if you want to interactively confirm what cpan
should download and install."

I've CCed him should you have any questions.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette