LLVM code generation can attempt to perform a load from a variable in the next condition of an expression under certain circumstances, thus turning the following condition:
if ( system_state < SYS_STATE_active && opt_bootscrub == BOOTSCRUB_IDLE ) Into: 0xffff82d080223967 <+103>: cmpl $0x3,0x37b032(%rip) # 0xffff82d08059e9a0 <system_state> 0xffff82d08022396e <+110>: setb -0x29(%rbp) 0xffff82d080223972 <+114>: cmpl $0x2,0x228a8b(%rip) # 0xffff82d08044c404 <opt_bootscrub> Such code will trigger a page fault if system_state >= SYS_STATE_active. In order to prevent such optimization signal to the compiler that accessing opt_bootscrub can have side effects by using ACCESS_ONCE. This has been reported and discussed with upstream LLVM: https://bugs.llvm.org/show_bug.cgi?id=39707 I haven't been able to find any other instances of such conditional expression that uses system_state together with an init variable or function. Signed-off-by: Roger Pau Monné <roger....@citrix.com> --- Cc: Andrew Cooper <andrew.coop...@citrix.com> Cc: George Dunlap <george.dun...@eu.citrix.com> Cc: Ian Jackson <ian.jack...@eu.citrix.com> Cc: Jan Beulich <jbeul...@suse.com> Cc: Julien Grall <julien.gr...@arm.com> Cc: Konrad Rzeszutek Wilk <konrad.w...@oracle.com> Cc: Stefano Stabellini <sstabell...@kernel.org> Cc: Tim Deegan <t...@xen.org> Cc: Wei Liu <wei.l...@citrix.com> --- xen/common/page_alloc.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c index 08ee8cfbb9..60adf6f64b 100644 --- a/xen/common/page_alloc.c +++ b/xen/common/page_alloc.c @@ -1772,7 +1772,17 @@ static void init_heap_pages( first_valid_mfn = mfn_min(page_to_mfn(pg), first_valid_mfn); spin_unlock(&heap_lock); - if ( system_state < SYS_STATE_active && opt_bootscrub == BOOTSCRUB_IDLE ) + if ( system_state < SYS_STATE_active && + /* + * Use ACCESS_ONCE in order to let the compiler know accessing + * opt_bootscrub in this context can have side-effects (since it + * might be unmapped depending on the value of system_state). + * This prevents the compiler from attempting a load of + * opt_bootscrub before checking the value of system_state. See: + * + * https://bugs.llvm.org/show_bug.cgi?id=39707 + */ + ACCESS_ONCE(opt_bootscrub) == BOOTSCRUB_IDLE ) idle_scrub = true; for ( i = 0; i < nr_pages; i++ ) -- 2.19.1 _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
