On Mon, Oct 31, 2016 at 04:03:44PM +0000, Suzuki K Poulose wrote:
> The hypervisor may not have full access to the kernel data structures
> and hence cannot safely use cpus_have_cap() helper for checking the
> system capability. Add a safe helper for hypervisors to check a constant
> system capability, which *doesn't* fall back to checking the bitmap
> maintained by the kernel.
>
> Cc: Marc Zyngier <marc.zyng...@arm.com>
> Cc: Catalin Marinas <catalin.mari...@arm.com>
> Cc: Will Deacon <will.dea...@arm.com>
> Signed-off-by: Suzuki K Poulose <suzuki.poul...@arm.com>
> ---
> arch/arm64/include/asm/cpufeature.h | 16 +++++++++++++---
> 1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/include/asm/cpufeature.h
> b/arch/arm64/include/asm/cpufeature.h
> index 758d74f..ae5e994 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -9,8 +9,6 @@
> #ifndef __ASM_CPUFEATURE_H
> #define __ASM_CPUFEATURE_H
>
> -#include <linux/jump_label.h>
> -
> #include <asm/hwcap.h>
> #include <asm/sysreg.h>
>
> @@ -45,6 +43,8 @@
>
> #ifndef __ASSEMBLY__
>
> +#include <linux/bug.h>
> +#include <linux/jump_label.h>
> #include <linux/kernel.h>
>
> /* CPU feature register tracking */
> @@ -122,6 +122,16 @@ static inline bool cpu_have_feature(unsigned int num)
> return elf_hwcap & (1UL << num);
> }
>
> +/* System capability check for constant caps */
> +static inline bool cpus_have_const_cap(int num)
> +{
> + if (__builtin_constant_p(num))
> + return static_branch_unlikely(&cpu_hwcap_keys[num]);
> + BUILD_BUG();
I think you'll already get a build failure if you pass a non-const num
to static_branch_unlikely, so this seems unnecessary. Furthermore, if
we're going to introduce a "const-only" version of this function, maybe
it's best to kill the __builtin_constant_p checks altogether, including
in the existing cpus_have_cap code? That way, the caller can make the
decision about whether or not they want to use static keys.
> + /* unreachable */
> + return false;
> +}
> +
> static inline bool cpus_have_cap(unsigned int num)
> {
> if (num >= ARM64_NCAPS)
It seems odd to check aginst ARM64_NCAPS here, but not in your new function.
Is the check actually necessary in either case? If so, we should probably
duplicate it for consistency.
Will
