linux-kernel - Re: Redoing eXclusive Page Frame Ownership (XPFO) with isolated CPUs in mind (for KVM to isolate its guests per CPU)

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Message-ID: <80a75259-e38b-be94-dc4a-827eddfae931@oracle.com>
Date:   Fri, 7 Sep 2018 15:30:10 -0600
From:   Khalid Aziz <khalid.aziz@...cle.com>
To:     Julian Stecklina <jsteckli@...zon.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     David Woodhouse <dwmw@...zon.co.uk>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        juerg.haefliger@....com, deepa.srinivasan@...cle.com,
        Jim Mattson <jmattson@...gle.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        linux-mm <linux-mm@...ck.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        joao.m.martins@...cle.com, pradeep.vincent@...cle.com,
        Andi Kleen <ak@...ux.intel.com>, kanth.ghatraju@...cle.com,
        Liran Alon <liran.alon@...cle.com>,
        Kees Cook <keescook@...gle.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        chris.hyser@...cle.com, Tyler Hicks <tyhicks@...onical.com>,
        John Haxby <john.haxby@...cle.com>,
        Jon Masters <jcm@...hat.com>
Subject: Re: Redoing eXclusive Page Frame Ownership (XPFO) with isolated CPUs
 in mind (for KVM to isolate its guests per CPU)

On 08/30/2018 10:00 AM, Julian Stecklina wrote:
> Hey everyone,
> 
> On Mon, 20 Aug 2018 15:27 Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>> On Mon, Aug 20, 2018 at 3:02 PM Woodhouse, David <dwmw@...zon.co.uk> wrote:
>>>
>>> It's the *kernel* we don't want being able to access those pages,
>>> because of the multitude of unfixable cache load gadgets.
>>
>> Ahh.
>>
>> I guess the proof is in the pudding. Did somebody try to forward-port
>> that patch set and see what the performance is like?
> 
> 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.
> 
> There were no problems in forward-porting the patch set to master.
> You can find the result here, including a patch makes the TLB flushing
> configurable:
> http://git.infradead.org/users/jsteckli/linux-xpfo.git/shortlog/refs/heads/xpfo-master
> 
> It survived some casual stress-ng runs. I can rerun the benchmarks on
> this version, but I doubt there is any change.
> 
>> It used to be just 500 LOC. Was that because they took horrible
>> shortcuts?
> 
> The patch is still fairly small. As for the horrible shortcuts, I let
> others comment on that.


Looks like the performance impact can be whole lot worse. On my test 
system with 2 Xeon Platinum 8160 (HT enabled) CPUs and 768 GB of memory, 
I am seeing very high penalty with XPFO when building 4.18.6 kernel 
sources with "make -j60":

              No XPFO patch   XPFO patch(No TLB flush)  XPFO(TLB Flush)
sys time      52m 54.036s       55m 47.897s              434m 8.645s

That is ~8% worse with TLB flush disabled and ~720% worse with TLB flush 
enabled. This test was with kernel sources being compiled on an ext4 
filesystem. XPFO seems to affect ext2 even more. With ext2 filesystem, 
impact was ~18.6% and ~900%.

--
Khalid