Re: [Xen-devel] [PATCH] Choose retpoline only when it is safe to use

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 06/02/2018 08:43, Zhenzhong Duan wrote:
> When ( ibrs && thunk == THUNK_DEFAULT && !retpoline_safe() ) is true,
> thunk is set to THUNK_JMP rather than THUNK_RETPOLINE.
>
> When (!ibrs && thunk == THUNK_DEFAULT && !retpoline_safe() ) is true,
> we should do the same.
>
> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx>

Why?  What improvement is this intended to give?

(IOW, the logic here is deliberate, and has a specific purpose.)

~Andrew

> ---
>  xen/arch/x86/spec_ctrl.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
> index f10ffbf..ab4b244 100644
> --- a/xen/arch/x86/spec_ctrl.c
> +++ b/xen/arch/x86/spec_ctrl.c
> @@ -247,10 +247,10 @@ void __init init_speculation_mitigations(void)
>  
>      /*
>       * If there are still no thunk preferences, the compiled default is
> -     * actually retpoline, and it is better than nothing.
> +     * actually retpoline, and it is better than nothing if it's retpoline 
> safe.
>       */
>      if ( thunk == THUNK_DEFAULT )
> -        thunk = THUNK_RETPOLINE;
> +        thunk = retpoline_safe() ? THUNK_RETPOLINE : THUNK_JMP;
>  
>      /* Apply the chosen settings. */
>      if ( thunk == THUNK_LFENCE )


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