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

On Thu, Nov 22, 2018 at 01:03:27PM +0100, 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>
> 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>

Acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>

I think long term we should invent / adopt some systematical approach,
but this will do for now.

I have written a QEMU based smoke test which will hopefully help us
catch further issues quickly.


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