[PATCH 4/4] x86/PoD: Command line option to prohibit any PoD operations

[Andrew Cooper <andrew.cooper3@citrix.com>]

PoD is only needed to cover a corner case with memory overcommit
(rebooting a ballooned down VM with insufficient host RAM available).

Its use comes with a performance hit, and inoperability with other
technologies such as PCI Passthrough.

Offer a command line option for administrators who want to be certain
that PoD is not in use for their VMs.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: George Dunlap <george.dunlap@eu.citrix.com>
---
 docs/misc/xen-command-line.markdown | 8 ++++++++
 xen/arch/x86/hvm/hvm.c              | 3 +++
 xen/arch/x86/mm.c                   | 6 ++++++
 xen/common/memory.c                 | 4 ++++
 xen/include/asm-x86/hvm/hvm.h       | 2 ++
 5 files changed, 23 insertions(+)

diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
index 416e559..aaf8a4f 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1173,6 +1173,14 @@ This option can be specified more than once (up to 8 times at present).
 ### ple\_window
 > `= <integer>`
 
+### pod
+> `= <boolean>`
+
+Default: `true`
+
+Permit or deny the use of Populate on Demand with x86 HVM guests.  If
+disabled, attempts to create VMs with `memory < maxmem` will fail.
+
 ### psr (Intel)
 > `= List of ( cmt:<boolean> | rmid_max:<integer> | cat:<boolean> | cos_max:<integer> | cdp:<boolean> )`
 
diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 21f42a7..cf895a7 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -92,6 +92,9 @@ unsigned long __section(".bss.page_aligned")
 static bool_t __initdata opt_hap_enabled = 1;
 boolean_param("hap", opt_hap_enabled);
 
+bool_t opt_pod_enabled = 1;
+boolean_param("pod", opt_pod_enabled);
+
 #ifndef opt_hvm_fep
 bool_t opt_hvm_fep;
 boolean_param("hvm_fep", opt_hvm_fep);
diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 92df36f..90270ba 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -4929,6 +4929,12 @@ long arch_memory_op(unsigned long cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
 
         if ( cmd == XENMEM_set_pod_target )
         {
+            if ( unlikely(!opt_pod_enabled) )
+            {
+                rc = -EOPNOTSUPP;
+                goto pod_target_out_unlock;
+            }
+
             if ( target.target_pages > d->max_pages )
             {
                 rc = -EINVAL;
diff --git a/xen/common/memory.c b/xen/common/memory.c
index a3bffb7..f5ed66e 100644
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -818,6 +818,10 @@ long do_memory_op(unsigned long cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
         if ( unlikely(start_extent >= reservation.nr_extents) )
             return start_extent;
 
+        if ( unlikely(!opt_pod_enabled) &&
+             (reservation.mem_flags & XENMEMF_populate_on_demand) )
+            return start_extent;
+
         d = rcu_lock_domain_by_any_id(reservation.domid);
         if ( d == NULL )
             return start_extent;
diff --git a/xen/include/asm-x86/hvm/hvm.h b/xen/include/asm-x86/hvm/hvm.h
index 0cf7da1..0341ba6 100644
--- a/xen/include/asm-x86/hvm/hvm.h
+++ b/xen/include/asm-x86/hvm/hvm.h
@@ -555,6 +555,8 @@ extern bool_t opt_hvm_fep;
 #define opt_hvm_fep 0
 #endif
 
+extern bool_t opt_pod_enabled;
+
 /* updates the current hardware p2m */
 void altp2m_vcpu_update_p2m(struct vcpu *v);
 
-- 
2.1.4