[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [xen stable-4.16] x86/shadow: fix and improve sh_page_has_multiple_shadows()
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) -- generated by git-patchbot for /home/xen/git/xen.git#stable-4.16
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