Hi Andrew,
On 11/22/18 12:23 PM, Andrew Cooper wrote:
On 22/11/2018 12:03, Roger Pau Monne wrote:
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>
You're actually missing two pieces here. If I recall the disassembly
correctly, there was a `sete %al`, and an `and %al, -0x29(%rbp)` which
had the cumulative effect of calculating `idle_scrub` branchlessly
(which is no doubt the intended optimisation).
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>
To unblock the Clang build, Acked-by: Andrew Cooper
<andrew.coop...@citrix.com>, although ideally with the expanded disassembly.
Actually is it enough in all the case? We are only preventing the
re-ordering for the compiler. The processor is still free to re-order it
and load opt_bootscrub before loading system_state.
Furthermore, this is relying on system_state to always be visible before
the init section is freed. I am not entirely whether we have the
explicit barrier for that.
I still think that longterm, we need to reconsider our position of
referencing __init things from non-__init functions.
+1.
Cheers,
--
Julien Grall
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