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

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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).

Default behaviour is unchanged.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>

---
Is module_param() a good thing for this? Other xen-balloon parameters are
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?).
---
 drivers/xen/Kconfig           |  5 ++++-
 drivers/xen/mem-reservation.c |  7 +++++++
 include/xen/mem-reservation.h | 11 ++++++++---
 3 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
index b459edfacff3..7b2c771e1813 100644
--- a/drivers/xen/Kconfig
+++ b/drivers/xen/Kconfig
@@ -87,7 +87,10 @@ config XEN_SCRUB_PAGES
 	  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. It can be disabled with
+	  mem_reservation.xen_scrub_pages=0 and also controlled at runtime with
+	  /sys/module/mem_reservation/parameters/xen_scrub_pages.
+
 	  If in doubt, say yes.
 
 config XEN_DEV_EVTCHN
diff --git a/drivers/xen/mem-reservation.c b/drivers/xen/mem-reservation.c
index 084799c6180e..5f08e19b6139 100644
--- a/drivers/xen/mem-reservation.c
+++ b/drivers/xen/mem-reservation.c
@@ -14,6 +14,13 @@
 
 #include <xen/interface/memory.h>
 #include <xen/mem-reservation.h>
+#include <linux/moduleparam.h>
+
+#ifdef CONFIG_XEN_SCRUB_PAGES
+bool __read_mostly xen_scrub_pages = true;
+module_param(xen_scrub_pages, bool, 0644);
+MODULE_PARM_DESC(xen_scrub_pages, "Scrub ballooned pages before giving them back to Xen");
+#endif
 
 /*
  * Use one extent per PAGE_SIZE to avoid to break down the page into
diff --git a/include/xen/mem-reservation.h b/include/xen/mem-reservation.h
index 80b52b4945e9..70c08f95bc84 100644
--- a/include/xen/mem-reservation.h
+++ b/include/xen/mem-reservation.h
@@ -17,12 +17,17 @@
 
 #include <xen/page.h>
 
+#ifdef CONFIG_XEN_SCRUB_PAGES
+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);
 }
+#else
+static inline void xenmem_reservation_scrub_page(struct page *page) { }
+#endif
 
 #ifdef CONFIG_XEN_HAVE_PVMMU
 void __xenmem_reservation_va_mapping_update(unsigned long count,
-- 
2.17.1