x86: make the dom0_max_vcpus option more flexible

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."

Allow a range for 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=4-8":

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

Existing command lines with "dom0_max_vcpus=N" still work as before
(and are equivalent to dom0_max_vcpus=N-N).

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Committed-by: Jan Beulich <jbeulich@suse.com>

1 files changed, 26 insertions, 3 deletions

diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
index eff930f7f8..92fa8fcd26 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -378,10 +378,33 @@ Specify the bit width of the DMA heap.
 Specify a list of IO ports to be excluded from dom0 access.
 
 ### dom0\_max\_vcpus
-> `= <integer>`
 
-Specify the maximum number of vcpus to give to dom0.  This defaults
-to the number of pcpus on the host.
+Either:
+
+> `= <integer>`.
+
+The 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:
+
+> `= <min>-<max>` where `<min>` and `<max>` are integers.
+
+Gives dom0 a number of VCPUs equal to the number of PCPUs, but always
+at least `<min>` and no more than `<max>`.  Using `<min>` may give
+more VCPUs than PCPUs.  `<min>` or `<max>` may be omitted and the
+defaults of 1 and unlimited respectively are used instead.
+
+For example, with `dom0_max_vcpus=4-8`:
+
+     Number of
+  PCPUs | Dom0 VCPUs
+   2    |  4
+   4    |  4
+   6    |  6
+   8    |  8
+  10    |  8
 
 ### dom0\_mem
 > `= List of ( min:<size> | max:<size> | <size> )`