Subject: [PATCH v3 11/14] ARM64: KVM: fix vgic_bitmap_get_reg function for BE 64bit case

From: [email-masked] (Christoffer Dall)
Date: Mon, 26 May 2014 17:49:10 +0200

On Tue, May 13, 2014 at 09:14:03AM -0700, Victor Kamensky wrote:
> Fix vgic_bitmap_get_reg function to return 'right' word address of
> 'unsigned long' bitmap value in case of BE 64bit image.
> 
> Signed-off-by: Victor Kamensky <[email-masked]>
> ---
>  virt/kvm/arm/vgic.c | 24 ++++++++++++++++++++++--
>  1 file changed, 22 insertions(+), 2 deletions(-)
> 
> diff --git a/virt/kvm/arm/vgic.c b/virt/kvm/arm/vgic.c
> index 529c336..d0a1a24 100644
> --- a/virt/kvm/arm/vgic.c
> +++ b/virt/kvm/arm/vgic.c
> @@ -101,14 +101,34 @@ static u32 vgic_nr_lr;
>  
>  static unsigned int vgic_maint_irq;
>  
> +/*
> + * struct vgic_bitmap contains union that provides two view of

it contains two unions, so:

...contains unions that provide

s/two view/two views/

> + * the same data. In one case it is an array of registers of
> + * u32 type (.reg), and in the other case it is a bitmape, which is

s/bitmape/bitmap/

> + * an array of 'unsgined long' (.reg_ul). It all works well in
> + * case of 32-bit (u32 and 'unsigned long' have the same size).

"in case of 32-bit" is too vague.  Write 'on 32-bit systems' if that's
what you mean.

> + * It alos works well in 64-bit LE, but breaks on 64-bit BE.

s/alos/also/

same with "in 64-bit" as above.

> + * In this case word sized register of even index actually resides

the word sized register

> + * in least significant word of 'unsigned long' which has address

in the least significant word of the 'unsigned long' at the register
offset plus 4 bytes.

> + * at offset plus 4 bytes. And word sized register of odd index

the 

> + * resides at most significant of 'unsigned long' which has offset
> + * minus 4 bytes. Define REG_OFFSET_SWIZZLE that would help to
> + * change offset of register in case of BE 64bit system.
> + */

I actually think this is quite hard to understand, how about:

/*
 * struct vgic_bitmap contains unions that provide two views of
 * the same data. In one case it is an array of registers of
 * u32's, and in the other case it is a bitmap of unsigned
 * longs.
 *
 * This does not work on 64-bit BE systems, because the bitmap access
 * will store two consecutive 32-bit words with the higher-addressed
 * register's bits at the lower index and the lower-addressed register's
 * bits at the higher index.
 *
 * Therefore, swizzle the register index when accessing the 32-bit word
 * registers to access the right register's value.
 */

> +#if defined(CONFIG_CPU_BIG_ENDIAN) && BITS_PER_LONG == 64
> +#define REG_OFFSET_SWIZZLE	1
> +#else
> +#define REG_OFFSET_SWIZZLE	0
> +#endif
> +
>  static u32 *vgic_bitmap_get_reg(struct vgic_bitmap *x,
>  				int cpuid, u32 offset)
>  {
>  	offset >>= 2;
>  	if (!offset)
> -		return x->percpu[cpuid].reg;
> +		return x->percpu[cpuid].reg + (offset ^ REG_OFFSET_SWIZZLE);
>  	else
> -		return x->shared.reg + offset - 1;
> +		return x->shared.reg + ((offset - 1) ^ REG_OFFSET_SWIZZLE);
>  }
>  
>  static int vgic_bitmap_get_irq_val(struct vgic_bitmap *x,
> -- 
> 1.8.1.4
>