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Date: Thu, 13 Sep 2018 08:11:49 +0200
From: Juerg Haefliger <>
To: Julian Stecklina <>
Cc: Linus Torvalds <>, David Woodhouse <>, 
	Konrad Rzeszutek Wilk <>,, 
	Jim Mattson <>, Andrew Cooper <>, 
	Linux Kernel Mailing List <>, Boris Ostrovsky <>, 
	linux-mm <>, Thomas Gleixner <>,,, Andi Kleen <>, 
	Khalid Aziz <>,, 
	Liran Alon <>, Kees Cook <>, 
	Kernel Hardening <>,, 
	Tyler Hicks <>, John Haxby <>, 
	Jon Masters <>
Subject: Re: Redoing eXclusive Page Frame Ownership (XPFO) with isolated CPUs
 in mind (for KVM to isolate its guests per CPU)

On Wed, Sep 12, 2018 at 5:37 PM, Julian Stecklina <> wrote:
> Julian Stecklina <> writes:
>> Linus Torvalds <> writes:
>>> On Fri, Aug 31, 2018 at 12:45 AM Julian Stecklina <> 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.
>> Well, it's at least in a range where it doesn't look hopeless.
>>> Kernel bullds are 90% user space at least for me, so a 2-3% slowdown
>>> from a kernel is not some small unnoticeable thing.
>> The overhead seems to come from the hooks that XPFO adds to
>> alloc/free_pages. These hooks add a couple of atomic operations per
>> allocated (4K) page for book keeping. Some of these atomic ops are only
>> for debugging and could be removed. There is also some opportunity to
>> streamline the per-page space overhead of XPFO.
> I've updated my XPFO branch[1] to make some of the debugging optional
> and also integrated the XPFO bookkeeping with struct page, instead of
> requiring CONFIG_PAGE_EXTENSION, which removes some checks in the hot
> path.

FWIW, that was my original design but there was some resistance to
adding more to the page struct and page extension was suggested

> These changes push the overhead down to somewhere between 1.5 and
> 2% for my quad core box in kernel compile. This is close to the
> measurement noise, so I take suggestions for a better benchmark here.
> Of course, if you hit contention on the xpfo spinlock then performance
> will suffer. I guess this is what happened on Khalid's large box.
> I'll try to remove the spinlocks and add fixup code to the pagefault
> handler to see whether this improves the situation on large boxes. This
> might turn out to be ugly, though.

I'm wondering how much performance we're loosing by having to split
hugepages. Any chance this can be quantified somehow? Maybe we can
have a pool of some sorts reserved for userpages and group allocations
so that we can track the XPFO state at the hugepage level instead of
at the 4k level to prevent/reduce page splitting. Not sure if that
causes issues or has any unwanted side effects though...


> Julian
> [1]
> --
> Amazon Development Center Germany GmbH
> Berlin - Dresden - Aachen
> main office: Krausenstr. 38, 10117 Berlin
> Geschaeftsfuehrer: Dr. Ralf Herbrich, Christian Schlaeger
> Ust-ID: DE289237879
> Eingetragen am Amtsgericht Charlottenburg HRB 149173 B

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