[xen stable-4.16] x86/shadow: fix and improve sh_page_has_multiple_shadows()

[patchbot@xxxxxxx]

commit cab866ee62d860e9ff4abe701163972d4e9f896d
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Fri Mar 31 08:38:42 2023 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Mar 31 08:38:42 2023 +0200

    x86/shadow: fix and improve sh_page_has_multiple_shadows()
    
    While no caller currently invokes the function without first making sure
    there is at least one shadow [1], we'd better eliminate UB here:
    find_first_set_bit() requires input to be non-zero to return a well-
    defined result.
    
    Further, using find_first_set_bit() isn't very efficient in the first
    place for the intended purpose.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    
    [1] The function has exactly two uses, and both are from OOS code, which
        is HVM-only. For HVM (but not for PV) sh_mfn_is_a_page_table(),
        guarding the call to sh_unsync(), guarantees at least one shadow.
        Hence even if sh_page_has_multiple_shadows() returned a bogus value
        when invoked for a PV domain, the subsequent is_hvm_vcpu() and
        oos_active checks (the former being redundant with the latter) will
        compensate. (Arguably that oos_active check should come first, for
        both clarity and efficiency reasons.)
    master commit: 2896224a4e294652c33f487b603d20bd30955f21
    master date: 2023-03-24 11:07:08 +0100
---
 xen/arch/x86/mm/shadow/private.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/arch/x86/mm/shadow/private.h b/xen/arch/x86/mm/shadow/private.h
index 738214f75e..762214f73c 100644
--- a/xen/arch/x86/mm/shadow/private.h
+++ b/xen/arch/x86/mm/shadow/private.h
@@ -324,7 +324,7 @@ static inline int sh_page_has_multiple_shadows(struct 
page_info *pg)
         return 0;
     shadows = pg->shadow_flags & SHF_page_type_mask;
     /* More than one type bit set in shadow-flags? */
-    return ( (shadows & ~(1UL << find_first_set_bit(shadows))) != 0 );
+    return shadows && (shadows & (shadows - 1));
 }
 
 #if (SHADOW_OPTIMIZATIONS & SHOPT_OUT_OF_SYNC)
--
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