[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] [PATCH] mm: fix LLVM code-generation issue

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 )


0xffff82d080223967 <+103>: cmpl   $0x3,0x37b032(%rip) # 0xffff82d08059e9a0 
0xffff82d08022396e <+110>: setb   -0x29(%rbp)
0xffff82d080223972 <+114>: cmpl   $0x2,0x228a8b(%rip) # 0xffff82d08044c404 

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 >=

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:


I haven't been able to find any other instances of such conditional
expression that uses system_state together with an init variable or

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

To unblock the Clang build, Acked-by: Andrew Cooper
<andrew.cooper3@xxxxxxxxxx>, 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.



Julien Grall

Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.