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Date:   Thu, 6 Sep 2018 22:31:28 +0200
From:   Marek =?utf-8?Q?Marczykowski-G=C3=B3recki?= 
        <marmarek@invisiblethingslab.com>
To:     Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc:     xen-devel@lists.xenproject.org, Juergen Gross <jgross@suse.com>,
        open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] xen/balloon: add runtime control for scrubbing ballooned
 out pages
Message-ID: <20180906203128.GJ1353@mail-itl>
References: <20180906153355.25363-1-marmarek@invisiblethingslab.com>
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On Thu, Sep 06, 2018 at 04:14:50PM -0400, Boris Ostrovsky wrote:
> On 09/06/2018 11:33 AM, Marek Marczykowski-G=C3=B3recki 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=3D significantly lower than maxmem=3D, 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).
> >
> > Default behaviour is unchanged.
> >
> > Signed-off-by: Marek Marczykowski-G=C3=B3recki <marmarek@invisiblething=
slab.com>
> >
> > ---
> > Is module_param() a good thing for this? Other xen-balloon parameters a=
re
> > in /sys/devices/system/xen_memory, so maybe it would make sense to put
> > this one there too? But then, cmdline parameter would need to be added
> > separately and comment about core_param() suggests it shouldn't be used
> > if not absolutely necessary (is it?).
>=20
>=20
> You can also use cmdline_find_option() in the balloon driver.
>=20
> I would prefer that all tunables for the balloon driver live in the same
> place. (Note that in that case
> Documentation/ABI/stable/sysfs-devices-system-xen_memory will need to be
> updated).

Ok, will move that. This will also mean that it will affect only balloon
driver, not other users of mem-reservation.c (grant allocator). But
given the reason for this tunable, I think that's fine.

--=20
Best Regards,
Marek Marczykowski-G=C3=B3recki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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