Re: [PATCH v3] xen/balloon: add runtime control for scrubbing ballooned out pages
From: Juergen Gross
Date: Mon Sep 10 2018 - 07:31:38 EST
On 07/09/18 18:49, Marek Marczykowski-GÃrecki 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
> Signed-off-by: Marek Marczykowski-GÃrecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>