[xen master] x86/mem-sharing: short-circuit p2m_is_shared() when MEM_SHARING=n

[patchbot@xxxxxxx]

commit 40bc520ca0f484163e440e3eb3c930c719ebf9f6
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Mon Apr 14 13:05:52 2025 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Apr 14 13:05:52 2025 +0200

    x86/mem-sharing: short-circuit p2m_is_shared() when MEM_SHARING=n
    
    Some of the uses of dom_cow aren't easily DCE-able (without extra
    #ifdef-ary), and hence it being constantly NULL when MEM_SHARING=n
    misguides Coverity into thinking that there may be a NULL deref in
    
            if ( p2m_is_shared(t) )
                d = dom_cow;
    
            if ( get_page(page, d) )
                return page;
    
    (in get_page_from_mfn_and_type()). Help the situation by making
    p2m_is_shared() be compile-time false when MEM_SHARING=n, thus also
    permitting the compiler to DCE some other code.
    
    Note that p2m_is_sharable() isn't used outside of mem_sharing.c, and
    hence P2M_SHARABLE_TYPES can simply be left undefined when
    MEM_SHARING=n.
    
    Coverity ID: 1645573
    Fixes: 79d91e178a1a ("dom_cow is needed for mem-sharing only")
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
---
 xen/arch/x86/include/asm/p2m.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/xen/arch/x86/include/asm/p2m.h b/xen/arch/x86/include/asm/p2m.h
index 6816a1be91..b9ce7d8705 100644
--- a/xen/arch/x86/include/asm/p2m.h
+++ b/xen/arch/x86/include/asm/p2m.h
@@ -136,11 +136,16 @@ typedef unsigned int p2m_query_t;
 #endif
 
 /* Shared types */
+#ifdef CONFIG_MEM_SHARING
 /* XXX: Sharable types could include p2m_ram_ro too, but we would need to
  * reinit the type correctly after fault */
 #define P2M_SHARABLE_TYPES (p2m_to_mask(p2m_ram_rw) \
                             | p2m_to_mask(p2m_ram_logdirty) )
 #define P2M_SHARED_TYPES   (p2m_to_mask(p2m_ram_shared))
+#else
+/* P2M_SHARABLE_TYPES deliberately not provided. */
+#define P2M_SHARED_TYPES 0
+#endif
 
 /* Types established/cleaned up via special accessors. */
 #define P2M_SPECIAL_TYPES (P2M_GRANT_TYPES | \
--
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