Re: [PATCH 1/2] xen/*/nospec: Provide common versions of evaluate_nospec/block_speculation

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.03.2024 17:10, Andrew Cooper wrote:
> --- a/xen/include/xen/nospec.h
> +++ b/xen/include/xen/nospec.h
> @@ -9,6 +9,29 @@
>  
>  #include <asm/nospec.h>
>  
> +/*
> + * Protect a conditional branch from bad speculation.  Architectures *must*
> + * provide arch_evaluate_nospec() for this to be effective.
> + */
> +static always_inline bool evaluate_nospec(bool cond)
> +{
> +#ifndef arch_evaluate_nospec
> +#define arch_evaluate_nospec(cond) cond

Hmm, noticed only while replying to patch 2: If the #define is to be kept
(see my reply there) it needs to be one of

#define arch_evaluate_nospec(cond) (cond)

or

#define arch_evaluate_nospec

Or it ought to be #undef-ed after use (thus preventing use in a context
where "cond" may expand to other than "cond").

Jan

> +#endif
> +    return arch_evaluate_nospec(cond);
> +}
> +
> +/*
> + * Halt speculation unconditonally.  Architectures *must* provide
> + * arch_block_speculation() for this to be effective.
> + */
> +static always_inline void block_speculation(void)
> +{
> +#ifdef arch_block_speculation
> +    arch_block_speculation();
> +#endif
> +}
> +
>  /**
>   * array_index_mask_nospec() - generate a ~0 mask when index < size, 0 
> otherwise
>   * @index: array element index