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From 6f2d89d68175e74aca9c67761aa87ffc8f5ffed1 Mon Sep 17 00:00:00 2001
From: Jan Beulich <jbeulich@suse.com>
Date: Fri, 31 Mar 2023 08:30:41 +0200
Subject: [PATCH 77/89] 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@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
[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 85bb26c7ea..c2bb1ed3c3 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)
--
2.40.0
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