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Subject: Re: [PATCH v3] xen/balloon: add runtime control for scrubbing
 ballooned out pages
To:     =?UTF-8?Q?Marek_Marczykowski-G=c3=b3recki?= 
        <marmarek@invisiblethingslab.com>, xen-devel@lists.xenproject.org
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Kai-Heng Feng <kai.heng.feng@canonical.com>,
        Thymo van Beers <thymovanbeers@gmail.com>,
        Jiri Kosina <jkosina@suse.cz>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        David Rientjes <rientjes@google.com>,
        open list <linux-kernel@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
References: <5B92A01D02000078001E67CE@prv1-mh.provo.novell.com>
 <20180907164912.21075-1-marmarek@invisiblethingslab.com>
From:   Juergen Gross <jgross@suse.com>
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Date:   Mon, 10 Sep 2018 13:31:30 +0200
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On 07/09/18 18:49, Marek Marczykowski-Górecki wrote:
> Scrubbing pages on initial balloon down can take some time, especially
> in nested virtualization case (nested EPT is slow). When HVM/PVH guest is
> started with memory= significantly lower than maxmem=, all the extra
> pages will be scrubbed before returning to Xen. But since most of them
> weren't used at all at that point, Xen needs to populate them first
> (from populate-on-demand pool). In nested virt case (Xen inside KVM)
> this slows down the guest boot by 15-30s with just 1.5GB needed to be
> returned to Xen.
> 
> Add runtime parameter to enable/disable it, to allow initially disabling
> scrubbing, then enable it back during boot (for example in initramfs).
> Such usage relies on assumption that a) most pages ballooned out during
> initial boot weren't used at all, and b) even if they were, very few
> secrets are in the guest at that time (before any serious userspace
> kicks in).
> Convert CONFIG_XEN_SCRUB_PAGES to CONFIG_XEN_SCRUB_PAGES_DEFAULT (also
> enabled by default), controlling default value for the new runtime
> switch.
> 
> Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>

Reviewed-by: Juergen Gross <jgross@suse.com>


Juergen