Switches of altp2m-s always expect a valid altp2m to be in place (and
indeed altp2m_vcpu_initialise() sets the active one to be at index 0).
The compiler, however, cannot know that, and hence it cannot eliminate
p2m_get_altp2m()'s case of returnin (literal) NULL. If then the compiler
decides to special case that code path in the caller, the dereference in
instances of

    atomic_dec(&p2m_get_altp2m(v)->active_vcpus);

can, to the code generator, appear to be NULL dereferences, leading to

In function 'atomic_dec',
    inlined from '...' at ...:
./arch/x86/include/asm/atomic.h:182:5: error: array subscript 0 is outside array bounds of 'int[0]' [-Werror=array-bounds=]

Aid the compiler by adding a BUG_ON() checking the return value of the
problematic p2m_get_altp2m(). Since with the use of the local variable
the 2nd p2m_get_altp2m() each will look questionable at the first glance
(Why is the local variable not used here?), open-code the only relevant
piece of p2m_get_altp2m() there.

To avoid repeatedly doing these transformations, and also to limit how
"bad" the open-coding really is, convert the entire operation to an
inline helper, used by all three instances (and accepting the redundant
BUG_ON(idx >= MAX_ALTP2M) in two of the three cases).

Reported-by: Charles Arnold <carnold@suse.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -4128,13 +4128,7 @@ void vmx_vmexit_handler(struct cpu_user_
             }
         }
 
-        if ( idx != vcpu_altp2m(v).p2midx )
-        {
-            BUG_ON(idx >= MAX_ALTP2M);
-            atomic_dec(&p2m_get_altp2m(v)->active_vcpus);
-            vcpu_altp2m(v).p2midx = idx;
-            atomic_inc(&p2m_get_altp2m(v)->active_vcpus);
-        }
+        p2m_set_altp2m(v, idx);
     }
 
     if ( unlikely(currd->arch.monitor.vmexit_enabled) )
--- a/xen/arch/x86/include/asm/p2m.h
+++ b/xen/arch/x86/include/asm/p2m.h
@@ -879,6 +879,26 @@ static inline struct p2m_domain *p2m_get
     return v->domain->arch.altp2m_p2m[index];
 }
 
+/* set current alternate p2m table */
+static inline bool p2m_set_altp2m(struct vcpu *v, unsigned int idx)
+{
+    struct p2m_domain *orig;
+
+    BUG_ON(idx >= MAX_ALTP2M);
+
+    if ( idx == vcpu_altp2m(v).p2midx )
+        return false;
+
+    orig = p2m_get_altp2m(v);
+    BUG_ON(!orig);
+    atomic_dec(&orig->active_vcpus);
+
+    vcpu_altp2m(v).p2midx = idx;
+    atomic_inc(&v->domain->arch.altp2m_p2m[idx]->active_vcpus);
+
+    return true;
+}
+
 /* Switch alternate p2m for a single vcpu */
 bool_t p2m_switch_vcpu_altp2m_by_id(struct vcpu *v, unsigned int idx);
 
--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -1789,13 +1789,8 @@ bool_t p2m_switch_vcpu_altp2m_by_id(stru
 
     if ( d->arch.altp2m_eptp[idx] != mfn_x(INVALID_MFN) )
     {
-        if ( idx != vcpu_altp2m(v).p2midx )
-        {
-            atomic_dec(&p2m_get_altp2m(v)->active_vcpus);
-            vcpu_altp2m(v).p2midx = idx;
-            atomic_inc(&p2m_get_altp2m(v)->active_vcpus);
+        if ( p2m_set_altp2m(v, idx) )
             altp2m_vcpu_update_p2m(v);
-        }
         rc = 1;
     }
 
@@ -2072,13 +2067,8 @@ int p2m_switch_domain_altp2m_by_id(struc
     if ( d->arch.altp2m_visible_eptp[idx] != mfn_x(INVALID_MFN) )
     {
         for_each_vcpu( d, v )
-            if ( idx != vcpu_altp2m(v).p2midx )
-            {
-                atomic_dec(&p2m_get_altp2m(v)->active_vcpus);
-                vcpu_altp2m(v).p2midx = idx;
-                atomic_inc(&p2m_get_altp2m(v)->active_vcpus);
+            if ( p2m_set_altp2m(v, idx) )
                 altp2m_vcpu_update_p2m(v);
-            }
 
         rc = 0;
     }

On 03/03/2023 7:31 am, Jan Beulich wrote:
> Switches of altp2m-s always expect a valid altp2m to be in place (and
> indeed altp2m_vcpu_initialise() sets the active one to be at index 0).
> The compiler, however, cannot know that, and hence it cannot eliminate
> p2m_get_altp2m()'s case of returnin (literal) NULL. If then the compiler
> decides to special case that code path in the caller, the dereference in
> instances of
>
>     atomic_dec(&p2m_get_altp2m(v)->active_vcpus);
>
> can, to the code generator, appear to be NULL dereferences, leading to
>
> In function 'atomic_dec',
>     inlined from '...' at ...:
> ./arch/x86/include/asm/atomic.h:182:5: error: array subscript 0 is outside array bounds of 'int[0]' [-Werror=array-bounds=]
>
> Aid the compiler by adding a BUG_ON() checking the return value of the
> problematic p2m_get_altp2m(). Since with the use of the local variable
> the 2nd p2m_get_altp2m() each will look questionable at the first glance
> (Why is the local variable not used here?), open-code the only relevant
> piece of p2m_get_altp2m() there.
>
> To avoid repeatedly doing these transformations, and also to limit how
> "bad" the open-coding really is, convert the entire operation to an
> inline helper, used by all three instances (and accepting the redundant
> BUG_ON(idx >= MAX_ALTP2M) in two of the three cases).
>
> Reported-by: Charles Arnold <carnold@suse.com>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>