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

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 <[e-mail]>
Reviewed-by: Juergen Gross <[e-mail]>
Signed-off-by: Boris Ostrovsky <[e-mail]>

Documentation/ABI/stable/sysfs-devices-system-xen_memory

···
 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:	[e-mail]
+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.

Documentation/admin-guide/kernel-parameters.txt

···
 			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>]]]

drivers/xen/Kconfig

···
 	  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

drivers/xen/mem-reservation.c

···
 
 #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

drivers/xen/xen-balloon.c

···
 #include <xen/xenbus.h>
 #include <xen/features.h>
 #include <xen/page.h>
+#include <xen/mem-reservation.h>
 
 #define PAGES2KB(_p) ((_p)<<(PAGE_SHIFT-10))
 
···
 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)
···
 	&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
 };
 

include/xen/mem-reservation.h

···
 
 #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