[PATCH] x86: make the dom0_max_vcpus option more flexible

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

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

The dom0_max_vcpus command line option only allows the exact number of
VCPUs for dom0 to be set.  It is not possible to say "up to N VCPUs
but no more than the number physically present."

Add min: and max: prefixes to the option to set a minimum number of
VCPUs, and a maximum which does not exceed the number of PCPUs.

For example, with "dom0_max_vcpus=min:4,max:8":

    PCPUs  Dom0 VCPUs
     2      4
     4      4
     6      6
     8      8
    10      8

The existing behaviour of "dom0_max_vcpus=N" still works as before.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
---
 docs/misc/xen-command-line.markdown |   29 +++++++++++++++++++++++++++--
 xen/arch/x86/domain_build.c         |   23 ++++++++++++++++++++++-
 2 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
index a6195f2..5f0c2cd 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -272,10 +272,35 @@ Specify the bit width of the DMA heap.
 
 ### dom0\_ioports\_disable
 ### dom0\_max\_vcpus
+
+Either:
+
 > `= <integer>`
 
-Specify the maximum number of vcpus to give to dom0.  This defaults
-to the number of pcpus on the host.
+The maximum number of VCPUs to give to dom0.  This number of VCPUs can
+be more than the number of PCPUs on the host.  The default is the
+number of PCPUs.
+
+Or:
+
+> `= List of ( min:<integer> | max:<integer> )`
+
+With the `min:` option dom0 will have at least this minimum number of
+VCPUs (default: 1).  This may be more than the number of PCPUs on the
+host.
+
+With the `max:` option dom0 will have a VCPUs for each PCPUs but no
+more than this maximum number (default: unlimited).
+
+For example, with `dom0_max_vcpus=min:4,max:8`:
+
+     Number of
+  PCPUs | Dom0 VCPUs
+   2    |  4
+   4    |  4
+   6    |  6
+   8    |  8
+  10    |  8
 
 ### dom0\_mem (ia64)
 > `= <size>`
diff --git a/xen/arch/x86/domain_build.c b/xen/arch/x86/domain_build.c
index b3c5d4c..5407f8d 100644
--- a/xen/arch/x86/domain_build.c
+++ b/xen/arch/x86/domain_build.c
@@ -83,7 +83,24 @@ static void __init parse_dom0_mem(const char *s)
 custom_param("dom0_mem", parse_dom0_mem);
 
 static unsigned int __initdata opt_dom0_max_vcpus;
-integer_param("dom0_max_vcpus", opt_dom0_max_vcpus);
+static unsigned int __initdata opt_dom0_max_vcpus_min = 1;
+static unsigned int __initdata opt_dom0_max_vcpus_max = UINT_MAX;
+
+static void __init parse_dom0_max_vcpus(const char *s)
+{
+    do {
+        if ( !strncmp(s, "min:", 4) )
+            opt_dom0_max_vcpus_min = simple_strtoul(s+4, &s, 0);
+        else if ( !strncmp(s, "max:", 4) )
+            opt_dom0_max_vcpus_max = simple_strtoul(s+4, &s, 0);
+        else
+            opt_dom0_max_vcpus = simple_strtoul(s, &s, 0);
+        if ( *s != ',' )
+            break;
+    } while ( *s++ == ',' );
+        
+}
+custom_param("dom0_max_vcpus", parse_dom0_max_vcpus);
 
 struct vcpu *__init alloc_dom0_vcpu0(void)
 {
@@ -91,6 +108,10 @@ struct vcpu *__init alloc_dom0_vcpu0(void)
         opt_dom0_max_vcpus = num_cpupool_cpus(cpupool0);
     if ( opt_dom0_max_vcpus > MAX_VIRT_CPUS )
         opt_dom0_max_vcpus = MAX_VIRT_CPUS;
+    if ( opt_dom0_max_vcpus_min > opt_dom0_max_vcpus )
+        opt_dom0_max_vcpus = opt_dom0_max_vcpus_min;
+    if ( opt_dom0_max_vcpus_max < opt_dom0_max_vcpus )
+        opt_dom0_max_vcpus = opt_dom0_max_vcpus_max;
 
     dom0->vcpu = xzalloc_array(struct vcpu *, opt_dom0_max_vcpus);
     if ( !dom0->vcpu )
-- 
1.7.2.5