Re: [Xen-devel] [PATCH] x86/spec_ctrl: Fix determination of when to use IBRS

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 06.02.18 at 14:48, <andrew.cooper3@xxxxxxxxxx> wrote:
> The original version of this logic was:
> 
>     /*
>      * On Intel hardware, we'd like to use retpoline in preference to
>      * IBRS, but only if it is safe on this hardware.
>      */
>     else if ( boot_cpu_has(X86_FEATURE_IBRSB) )
>     {
>         if ( retpoline_safe() )
>             thunk = THUNK_RETPOLINE;
>         else
>             ibrs = true;
>     }
> 
> but it was changed by a request during review.  Sadly, the result is buggy as
> it breaks the later fallback logic by allowing IBRS to appear as available
> when in fact it isn't.
> 
> This in practice means that on repoline-unsafe hardware without IBRS, we
> select THUNK_JUMP despite intending to select THUNK_RETPOLINE.
> 
> Reported-by: Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>



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