[xen master] x86/altp2m: help gcc13 to avoid it emitting a warning

[patchbot@xxxxxxx]

commit be62b1fc2aa7375d553603fca07299da765a89fe
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Mon Mar 13 15:16:21 2023 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Mar 13 15:16:21 2023 +0100

    x86/altp2m: help gcc13 to avoid it emitting a warning
    
    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@xxxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/hvm/vmx/vmx.c     |  8 +-------
 xen/arch/x86/include/asm/p2m.h | 20 ++++++++++++++++++++
 xen/arch/x86/mm/p2m.c          | 14 ++------------
 3 files changed, 23 insertions(+), 19 deletions(-)

diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index 278b829f73..00b531f76c 100644
--- 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_regs *regs)
             }
         }
 
-        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) )
diff --git a/xen/arch/x86/include/asm/p2m.h b/xen/arch/x86/include/asm/p2m.h
index bd684d02f3..cd43d8621a 100644
--- 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_altp2m(struct 
vcpu *v)
     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);
 
diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
index c5561fe221..7e481aec38 100644
--- 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(struct vcpu *v, 
unsigned int idx)
 
     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(struct domain *d, 
unsigned int idx)
     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;
     }
--
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