Re: [PATCH] KVM: PPC: Add generic single register ioctls

[Scott Wood <scottwood@freescale.com>]

On 01/05/2012 10:15 PM, Alexander Graf wrote:
> Right now we transfer a static struct every time we want to get or set
> registers. Unfortunately, over time we realize that there are more of
> these than we thought of before and the extensibility and flexibility of
> transferring a full struct every time is limited.
> 
> So this is a new approach to the problem. With these new ioctls, we can
> get and set a single register that is identified by an ID. This allows for
> very precise and limited transmittal of data. When we later realize that
> it's a better idea to shove over multiple registers at once, we can reuse
> most of the infrastructure and simply implement a GET_MANY_REGS / SET_MANY_REGS
> interface.
> 
> The only downpoint I see to this one is that it needs to pad to 1024 bits
> (hardware is already on 512 bit registers, so I wanted to leave some room)
> which is slightly too much for transmitting only 64 bits. But if that's all
> the tradeoff we have to do for getting an extensible interface, I'd say go
> for it nevertheless.

The comment about padding is no longer relevant.

> +/*
> + * Architecture specific registers are to be defined in arch headers and
> + * ORed with the arch identifier.
> + */
> +#define KVM_REG_PPC		0x1000000000000000ULL
> +#define KVM_REG_X86		0x2000000000000000ULL
> +#define KVM_REG_IA64		0x3000000000000000ULL
> +#define KVM_REG_ARM		0x4000000000000000ULL
> +#define KVM_REG_S390		0x5000000000000000ULL
> +
> +#define KVM_REG_SIZE_SHIFT	52
> +#define KVM_REG_SIZE_MASK	0x00f0000000000000ULL
> +#define KVM_REG_SIZE_U8		0x0000000000000000ULL
> +#define KVM_REG_SIZE_U16	0x0010000000000000ULL
> +#define KVM_REG_SIZE_U32	0x0020000000000000ULL
> +#define KVM_REG_SIZE_U64	0x0030000000000000ULL
> +#define KVM_REG_SIZE_U128	0x0040000000000000ULL
> +#define KVM_REG_SIZE_U256	0x0050000000000000ULL
> +#define KVM_REG_SIZE_U512	0x0060000000000000ULL
> +#define KVM_REG_SIZE_U1024	0x0070000000000000ULL

Why not just encode directly as number of bytes?

-Scott