Re: [PATCH] xen/arm: Using unsigned long for arm64 MPIDR mask

[Julien Grall <julien@xxxxxxx>]

Hi Wei,

On 05/01/2021 10:17, Wei Chen wrote:
> Curretly, Xen is using UINT32 for MPIDR mask to retrieve
s/Curretly/Currently/

> affinity[0,1,2,3] values for MPIDR_EL1 register. The value
> of MPIDR_EL1 is 64-bit unsigned long. The operation of 64-bit
> and 32-bit integers are compiler related. This means the value
> is unpredictable.
So I agree that ~MPIDR_AFF0_MASK will do the negation in 32-bit rather 
than 64-bit. However, I disagree that this is unpredicable or compiler 
specific.
> For example, when we are using MPIDR_AFF0_MASK to get
> cluster_id from a 64-bit integer in gic-v3 driver:
> uint64_t cluster_id = cpu_logical_map(cpu) & ~MPIDR_AFF0_MASK;
>
> When MPIDR_AFF0_MASK is UINT32, compiler output:
>      f7c: 92785c16 and x22, x0, #0xffffff00
> When MPIDR_AFF0_MASK is unsigned long, compiler output:
>      f88: 9278dc75 and x21, x3, #0xffffffffffffff00
>
> If we have a cpu_logical_map(cpu)= 0x1,00000000. We except
> to get a cluster_id 1, but with UINT32 MPIDR_AFF0_MASK, we
> will get 0.
Something doesn't match here. If the cluster_id were 1, then it should 
surely be 1 as well even with the 32-bit mask because there is no shift.
So did you intend to say 0x1,00000000?

> So, in this patch, we force aarch64 to use unsigned long
> as MPIDR mask to avoid such unpredictable operations.
Per above, I don't think this is unpredictable.

> Signed-off-by: Wei Chen <wei.chen@xxxxxxx>
> ---
>   xen/include/asm-arm/processor.h | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/xen/include/asm-arm/processor.h b/xen/include/asm-arm/processor.h
> index 87c8136022..5c1768cdec 100644
> --- a/xen/include/asm-arm/processor.h
> +++ b/xen/include/asm-arm/processor.h
> @@ -75,11 +75,11 @@
>   
>   /* MPIDR Multiprocessor Affinity Register */
>   #define _MPIDR_UP           (30)
> -#define MPIDR_UP            (_AC(1,U) << _MPIDR_UP)
> +#define MPIDR_UP            (_AC(1,UL) << _MPIDR_UP)
>   #define _MPIDR_SMP          (31)
> -#define MPIDR_SMP           (_AC(1,U) << _MPIDR_SMP)
> +#define MPIDR_SMP           (_AC(1,UL) << _MPIDR_SMP)
>   #define MPIDR_AFF0_SHIFT    (0)
> -#define MPIDR_AFF0_MASK     (_AC(0xff,U) << MPIDR_AFF0_SHIFT)
> +#define MPIDR_AFF0_MASK     (_AC(0xff,UL) << MPIDR_AFF0_SHIFT)
>   #ifdef CONFIG_ARM_64
>   #define MPIDR_HWID_MASK     _AC(0xff00ffffff,UL)
>   #else
--
Julien Grall