Re: [Xen-devel] [PATCH v2 11/19] xen/arm32: Use alternative to skip the check of pending serrors

[Julien Grall <julien.grall@xxxxxxx>]

Hi Wei,

On 30/03/17 10:13, Wei Chen wrote:
> We have provided an option to administrator to determine how to
> handle the SErrors. In order to skip the check of pending SError,
> in conventional way, we have to read the option every time before
> we try to check the pending SError. This will add overhead to check
> the option at every trap.
>
> The ARM32 supports the alternative patching feature. We can use an
> ALTERNATIVE to avoid checking option at every trap. We added a new
> cpufeature named "SKIP_CHECK_PENDING_VSERROR". This feature will be
> enabled when the option is not diverse.
>
> Signed-off-by: Wei Chen <Wei.Chen@xxxxxxx>
Reviewed-by: Julien Grall <julien.grall@xxxxxxx>

> ---
Stefano, this is requiring the alternative patch (see [1]) which is 
still in review.
Wei, a general rule is to mention the dependencies between series. So we 
don't apply by mistake in the wrong order.
Cheers,

[1] 
https://lists.xenproject.org/archives/html/xen-devel/2017-03/msg04132.html
>  xen/arch/arm/arm32/entry.S | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/xen/arch/arm/arm32/entry.S b/xen/arch/arm/arm32/entry.S
> index 2187226..105cae8 100644
> --- a/xen/arch/arm/arm32/entry.S
> +++ b/xen/arch/arm/arm32/entry.S
> @@ -1,5 +1,6 @@
>  #include <asm/asm_defns.h>
>  #include <asm/regs.h>
> +#include <asm/alternative.h>
>  #include <public/xen.h>
>
>  #define SAVE_ONE_BANKED(reg)    mrs r11, reg; str r11, [sp, #UREGS_##reg]
> @@ -44,6 +45,14 @@ save_guest_regs:
>          SAVE_BANKED(fiq)
>          SAVE_ONE_BANKED(R8_fiq); SAVE_ONE_BANKED(R9_fiq); 
> SAVE_ONE_BANKED(R10_fiq)
>          SAVE_ONE_BANKED(R11_fiq); SAVE_ONE_BANKED(R12_fiq);
> +
> +        /*
> +         * If the SKIP_CHECK_PENDING_VSERROR has been set in the cpu feature,
> +         * the checking of pending SErrors will be skipped.
> +         */
> +        ALTERNATIVE("nop",
> +                    "b skip_check",
> +                    SKIP_CHECK_PENDING_VSERROR)
>          /*
>           * Start to check pending virtual abort in the gap of Guest -> HYP
>           * world switch.
> @@ -99,6 +108,7 @@ abort_guest_exit_end:
>           */
>          bne return_from_trap
>
> +skip_check:
>          mov pc, lr
>
>  #define DEFINE_TRAP_ENTRY(trap)                                         \
--
Julien Grall

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