Re: [Tarantool-patches] [PATCH luajit] x64: Fix __call metamethod return dispatch.

[Sergey Kaplun <skaplun@tarantool.org>]

Hi, Igor!

Thanks for the patch!
LGTM except the single nit.

On 04.12.20, Igor Munkin wrote:
> From: Mike Pall <mike>
> 
> After linking new cframe to the chain KBASEa still stores the address of
> the previous one. If the execution proceeds to <lj_vmeta_call> KBASE
> value (i.e. low 32 bits of the stored address) might be equal to the
> current BASE address value so the execution takes the invalid path. Such
> address clashing occurs only on x86_64 platform with disabled LJ_GC64,

Typo: s/address/address parts/ (considering your message in PR).

> so 64-bit registers have to be compared in x64 build.
> 
> NB: Though there is only 32-bit load to restore BASE value prior to the
> comparison, the high 32 bits of RDX are reset to zeros, according to x86
> long mode semantics.
> 
> Igor Munkin:
> * backported the original patch to tarantool/luajit repo
> * extended the original commit message with the rationale
> 
> For more info and explanation see LuaJIT/LuaJIT#636.
> 
> Relates to tarantool/tarantool#4518
> Relates to tarantool/tarantool#4649
> 
> Signed-off-by: Igor Munkin <imun@tarantool.org>
> ---
> 
> Issues:
> * https://github.com/tarantool/tarantool/issues/4518
> * https://github.com/tarantool/tarantool/issues/4649
> Branch:
> * https://github.com/tarantool/luajit/tree/imun/gh-4518-cmp-64-bit-regs-in-vmeta-call
> 
> CI is kinda green, considering C6 EOL and the corresponding failures:
> * https://gitlab.com/tarantool/tarantool/-/pipelines/225349795
> 
> @ChangeLog:
> * Fixed address clashing occurring while __call metamethod dispatching
>   (gh-4518, gh-4649).
> 
> Unfortunately, there is neither test nor reproducer for this failure, so
> we'll know that the patch works only on production installations.
> 
>  src/vm_x86.dasc | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/src/vm_x86.dasc b/src/vm_x86.dasc
> index 56bee14..d76fbe3 100644
> --- a/src/vm_x86.dasc
> +++ b/src/vm_x86.dasc
> @@ -1374,7 +1374,11 @@ static void build_subroutines(BuildCtx *ctx)
>    |  mov LFUNC:RB, [RA-8]
>    |  add NARGS:RD, 1
>    |  // This is fragile. L->base must not move, KBASE must always be defined.
> +  |.if x64
> +  |  cmp KBASEa, rdx			// Continue with CALLT if flag set.
> +  |.else
>    |  cmp KBASE, BASE			// Continue with CALLT if flag set.
> +  |.endif
>    |  je ->BC_CALLT_Z
>    |  mov BASE, RA
>    |  ins_call				// Otherwise call resolved metamethod.
> -- 
> 2.25.0
> 

-- 
Best regards,
Sergey Kaplun