[PATCH] x86: set dom0's default maximum reservation to the initial number of pages

[David Vrabel <david.vrabel@citrix.com>]

From: David Vrabel <david.vrabel@citrix.com>

If a maximum reservation for dom0 is not explictly given (i.e., no
dom0_mem=max:MMM command line option), then set the maximum
reservation to the initial number of pages.  This is what most people
seem to expect when they specify dom0_mem=512M (i.e., exactly 512 MB
and no more).

This change means that with Linux 3.0.5 and later kernels,
dom0_mem=512M has the same result as older, 'classic Xen' kernels. The
older kernels used the initial number of pages to set the maximum
number of pages and did not query the hypervisor for the maximum
reservation.

It is still possible to have a larger reservation by explicitly
specifying dom0_mem=max:MMM.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
---
Keir,

Suggest waiting for an Ack from Konrad (I think it results in the
behaviour we want but would prefer it if Konrad confirmed).

Also consider for 4.1.

Thanks.

David

 docs/misc/xen-command-line.markdown |    8 +++++++-
 xen/arch/x86/domain_build.c         |   10 ++++++++++
 2 files changed, 17 insertions(+), 1 deletions(-)

diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
index beb8462..0798700 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -221,12 +221,18 @@ Specify the total size for dom0.
 ### dom0\_mem (x86)
 > `= List of ( min:<value> | max: <value> | <value> )`
 
-each `<value>` is a size parameter.  If the size is positive, it represents an absolute value.  If the size is negative, the size specified is subtracted from the total available memory.
+Specify the amount of memory for the initial domain (dom0) and the maximum reservation (the maximum amount of memory that dom0 can be increased or ballooned to).
+
+Each `<value>` is a size parameter.  If the size is positive, it represents an absolute value.  If the size is negative, the size specified is subtracted from the total available memory.
 
 * `min:<value>` specifies the minimum amount of memory allocated to dom0.
 * `max:<value>` specifies the maximum amount of memory allocated to dom0.
 * `<value>` specified the exact amount of memory allocated to dom0.
 
+If `max:<value>` is specified then this sets the maximum reservation, otherwise the maximum reservation is set to the amount of memory allocated to dom0.
+
+For example, with `dom0_mem=512M`, dom0 starts with 512 MB and cannot balloon up any more. With `dom0_mem=512M,max:2G`, dom0 starts with 512 MB of memory and can balloon up to 2 GB.
+
 ### dom0\_shadow
 ### dom0\_vcpus\_pin
 > `= <boolean>`
diff --git a/xen/arch/x86/domain_build.c b/xen/arch/x86/domain_build.c
index b3c5d4c..0c09abc 100644
--- a/xen/arch/x86/domain_build.c
+++ b/xen/arch/x86/domain_build.c
@@ -253,6 +253,16 @@ static unsigned long __init compute_dom0_nr_pages(
     }
 #endif
 
+    /*
+     * Set dom0's maximum reservation.
+     *
+     * If no maximum was set with dom0_mem=max:MMM, then the maximum
+     * is the same as the initial number of pages.  This is so
+     * dom0_mem=MMM gives the behaviour most people expect (i.e., this
+     * much RAM and no more).
+     */
+    if ( max_pages == LONG_MAX )
+        max_pages = nr_pages;
     d->max_pages = min_t(unsigned long, max_pages, UINT_MAX);
 
     return nr_pages;
-- 
1.7.2.5