Re: [UNIKRAFT PATCH 2/2] arch/arm: Provide ukarch_spinwait noop for ARM

[Julien Grall <julien@xxxxxxx>]

Hi,

On 23/07/2020 09:15, Alexander Jung wrote:
> ARM does not support a pause-type instruction for busy spin wait
> operations.  This commit provides the same alias to be used in
> instances where it is required but defaults to a blank operation.
On Arm, the closest instruction would be "yield".

> Signed-off-by: Alexander Jung <alexander.jung@xxxxxxxxx>
> ---
>   arch/arm/arm/include/uk/asm/lcpu.h   | 5 +++++
>   arch/arm/arm64/include/uk/asm/lcpu.h | 5 +++++
>   2 files changed, 10 insertions(+)
>
> diff --git a/arch/arm/arm/include/uk/asm/lcpu.h 
> b/arch/arm/arm/include/uk/asm/lcpu.h
> index cdeffc3..f38ffb9 100644
> --- a/arch/arm/arm/include/uk/asm/lcpu.h
> +++ b/arch/arm/arm/include/uk/asm/lcpu.h
> @@ -57,3 +57,8 @@ struct __regs {
>   #ifndef wmb
>   #define wmb() __asm__("dsb" : : : "memory")
>   #endif
> +
> +static inline void ukarch_spinwait(void)
> +{
> +       /* Intelligent busy wait not supported on arm. */
This at least need to contain a barrier() to force the compiler to 
reload the memory after the wait. Otherwise, it may optimize waiting 
loop to infinite loop.
Unfortunately, I don't have a setup with Unikraft. But I am happy to 
provide a patch if someone else can test it.
> +}
> diff --git a/arch/arm/arm64/include/uk/asm/lcpu.h 
> b/arch/arm/arm64/include/uk/asm/lcpu.h
> index 45d1b23..51e17ad 100644
> --- a/arch/arm/arm64/include/uk/asm/lcpu.h
> +++ b/arch/arm/arm64/include/uk/asm/lcpu.h
> @@ -146,4 +146,9 @@ static inline unsigned long ukarch_read_sp(void)
>         return sp;
>   }
>   
> +static inline void ukarch_spinwait(void)
> +{
> +       /* Intelligent busy wait not supported on arm64. */
Likewise here.

> +}
> +
>   #endif /* __ASSEMBLY__ */
Cheers,

--
Julien Grall