[PATCH v4 5/6] xen/mem_access: wrap memory access when VM_EVENT=n

[Penny Zheng <Penny.Zheng@xxxxxxx>]

Feature memory access is based on vm event subsystem, and it could be disabled
in the future. So a few switch-blocks in do_altp2m_op() need
vm_event_is_enabled() condition check to pass compilation when ALTP2M=y and
VM_EVENT=n(, hence MEM_ACCESS=n), like HVMOP_altp2m_set_mem_access, etc.
Function p2m_mem_access_check() still needs stub when VM_EVENT=n to
pass compilation.
Although local variable "req_ptr" still remains NULL throughout its lifetime,
with the change of NULL assignment, we will face runtime undefined error only
when CONFIG_USBAN is on. So we strengthen the condition check via adding
vm_event_is_enabled() for the special case.

Signed-off-by: Penny Zheng <Penny.Zheng@xxxxxxx>
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v1 -> v3:
- a comment next to the excessive condition
- use vm_event_is_enabled() instead
- avoid heavy churn by using the inverted condition plus break
---
v3 - v4:
- refine comment
---
 xen/arch/x86/hvm/hvm.c                | 26 +++++++++++++++++++++++++-
 xen/arch/x86/include/asm/mem_access.h | 10 ++++++++++
 2 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 07e54890d9..b34cd29629 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -52,6 +52,7 @@
 #include <asm/i387.h>
 #include <asm/mc146818rtc.h>
 #include <asm/mce.h>
+#include <asm/mem_access.h>
 #include <asm/monitor.h>
 #include <asm/msr.h>
 #include <asm/mtrr.h>
@@ -2082,7 +2083,12 @@ int hvm_hap_nested_page_fault(paddr_t gpa, unsigned long 
gla,
 #endif
     }
 
-    if ( req_ptr )
+    /*
+     * req_ptr being constant NULL when !CONFIG_VM_EVENT, CONFIG_UBSAN=y
+     * builds have been observed to still hit undefined-ness at runtime.
+     * Hence do a seemingly redundant vm_event_is_enabled() check here.
+     */
+    if ( req_ptr && vm_event_is_enabled(curr) )
     {
         if ( monitor_traps(curr, sync, req_ptr) < 0 )
             rc = 0;
@@ -4804,6 +4810,12 @@ static int do_altp2m_op(
         break;
 
     case HVMOP_altp2m_set_mem_access:
+        if ( !vm_event_is_enabled(current) )
+        {
+            rc = -EOPNOTSUPP;
+            break;
+        }
+
         if ( a.u.mem_access.pad )
             rc = -EINVAL;
         else
@@ -4813,6 +4825,12 @@ static int do_altp2m_op(
         break;
 
     case HVMOP_altp2m_set_mem_access_multi:
+        if ( !vm_event_is_enabled(current) )
+        {
+            rc = -EOPNOTSUPP;
+            break;
+        }
+
         if ( a.u.set_mem_access_multi.pad ||
              a.u.set_mem_access_multi.opaque > a.u.set_mem_access_multi.nr )
         {
@@ -4844,6 +4862,12 @@ static int do_altp2m_op(
         break;
 
     case HVMOP_altp2m_get_mem_access:
+        if ( !vm_event_is_enabled(current) )
+        {
+            rc = -EOPNOTSUPP;
+            break;
+        }
+
         if ( a.u.mem_access.pad )
             rc = -EINVAL;
         else
diff --git a/xen/arch/x86/include/asm/mem_access.h 
b/xen/arch/x86/include/asm/mem_access.h
index 257ed33de1..790bed81e8 100644
--- a/xen/arch/x86/include/asm/mem_access.h
+++ b/xen/arch/x86/include/asm/mem_access.h
@@ -14,6 +14,7 @@
 #ifndef __ASM_X86_MEM_ACCESS_H__
 #define __ASM_X86_MEM_ACCESS_H__
 
+#ifdef CONFIG_VM_EVENT
 /*
  * Setup vm_event request based on the access (gla is -1ull if not available).
  * Handles the rw2rx conversion. Boolean return value indicates if event type
@@ -25,6 +26,15 @@
 bool p2m_mem_access_check(paddr_t gpa, unsigned long gla,
                           struct npfec npfec,
                           struct vm_event_st **req_ptr);
+#else
+static inline bool p2m_mem_access_check(paddr_t gpa, unsigned long gla,
+                                        struct npfec npfec,
+                                        struct vm_event_st **req_ptr)
+{
+    *req_ptr = NULL;
+    return false;
+}
+#endif /* CONFIG_VM_EVENT */
 
 /* Check for emulation and mark vcpu for skipping one instruction
  * upon rescheduling if required. */
-- 
2.34.1