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xen/balloon: add runtime control for scrubbing ballooned out pages

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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>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
This commit is contained in:
Marek Marczykowski-Górecki 2018-09-07 18:49:08 +02:00 committed by Boris Ostrovsky
parent 87dffe86d4
commit 197ecb3802
6 changed files with 33 additions and 6 deletions
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9
Documentation/ABI/stable/sysfs-devices-system-xen_memory
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@ -75,3 +75,12 @@ Contact: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Description:
Amount (in KiB) of low (or normal) memory in the
balloon.
What: /sys/devices/system/xen_memory/xen_memory0/scrub_pages
Date: September 2018
KernelVersion: 4.20
Contact: xen-devel@lists.xenproject.org
Description:
Control scrubbing pages before returning them to Xen for others domains
use. Can be set with xen_scrub_pages cmdline
parameter. Default value controlled with CONFIG_XEN_SCRUB_PAGES_DEFAULT.

6
Documentation/admin-guide/kernel-parameters.txt
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@ -5000,6 +5000,12 @@
Disables the PV optimizations forcing the HVM guest to
run as generic HVM guest with no PV drivers.
xen_scrub_pages= [XEN]
Boolean option to control scrubbing pages before giving them back
to Xen, for use by other domains. Can be also changed at runtime
with /sys/devices/system/xen_memory/xen_memory0/scrub_pages.
Default value controlled with CONFIG_XEN_SCRUB_PAGES_DEFAULT.
xirc2ps_cs= [NET,PCMCIA]
Format:
<irq>,<irq_mask>,<io>,<full_duplex>,<do_sound>,<lockup_hack>[,<irq2>[,<irq3>[,<irq4>]]]

10
drivers/xen/Kconfig
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@ -79,15 +79,19 @@ config XEN_BALLOON_MEMORY_HOTPLUG_LIMIT
This value is used to allocate enough space in internal
tables needed for physical memory administration.
config XEN_SCRUB_PAGES
bool "Scrub pages before returning them to system"
config XEN_SCRUB_PAGES_DEFAULT
bool "Scrub pages before returning them to system by default"
depends on XEN_BALLOON
default y
help
Scrub pages before returning them to the system for reuse by
other domains. This makes sure that any confidential data
is not accidentally visible to other domains. Is it more
secure, but slightly less efficient.
secure, but slightly less efficient. This can be controlled with
xen_scrub_pages=0 parameter and
/sys/devices/system/xen_memory/xen_memory0/scrub_pages.
This option only sets the default value.
If in doubt, say yes.
config XEN_DEV_EVTCHN

4
drivers/xen/mem-reservation.c
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@ -14,6 +14,10 @@
#include <xen/interface/memory.h>
#include <xen/mem-reservation.h>
#include <linux/moduleparam.h>
bool __read_mostly xen_scrub_pages = IS_ENABLED(CONFIG_XEN_SCRUB_PAGES_DEFAULT);
core_param(xen_scrub_pages, xen_scrub_pages, bool, 0);
/*
* Use one extent per PAGE_SIZE to avoid to break down the page into

3
drivers/xen/xen-balloon.c
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@ -44,6 +44,7 @@
#include <xen/xenbus.h>
#include <xen/features.h>
#include <xen/page.h>
#include <xen/mem-reservation.h>
#define PAGES2KB(_p) ((_p)<<(PAGE_SHIFT-10))
@ -137,6 +138,7 @@ static DEVICE_ULONG_ATTR(schedule_delay, 0444, balloon_stats.schedule_delay);
static DEVICE_ULONG_ATTR(max_schedule_delay, 0644, balloon_stats.max_schedule_delay);
static DEVICE_ULONG_ATTR(retry_count, 0444, balloon_stats.retry_count);
static DEVICE_ULONG_ATTR(max_retry_count, 0644, balloon_stats.max_retry_count);
static DEVICE_BOOL_ATTR(scrub_pages, 0644, xen_scrub_pages);
static ssize_t show_target_kb(struct device *dev, struct device_attribute *attr,
char *buf)
@ -203,6 +205,7 @@ static struct attribute *balloon_attrs[] = {
&dev_attr_max_schedule_delay.attr.attr,
&dev_attr_retry_count.attr.attr,
&dev_attr_max_retry_count.attr.attr,
&dev_attr_scrub_pages.attr.attr,
NULL
};

7
include/xen/mem-reservation.h
View File

@ -17,11 +17,12 @@
#include <xen/page.h>
extern bool xen_scrub_pages;
static inline void xenmem_reservation_scrub_page(struct page *page)
{
#ifdef CONFIG_XEN_SCRUB_PAGES
clear_highpage(page);
#endif
if (xen_scrub_pages)
clear_highpage(page);
}
#ifdef CONFIG_XEN_HAVE_PVMMU