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

[Julien Grall <julien.grall@xxxxxxx>]

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.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.
+1.

Cheers,

--
Julien Grall

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