[RFC PATCH v2 10/23] KVM: arm64: Extend reset_unknown() to handle mixed RES0/UNKNOWN registers

[christoffer.dall@arm.com (Christoffer Dall)]

On Fri, Sep 28, 2018 at 02:39:14PM +0100, Dave Martin wrote:
> The reset_unknown() system register helper initialises a guest
> register to a distinctive junk value on vcpu reset, to help expose
> and debug deficient register initialisation within the guest.
> 
> Some registers such as the SVE control register ZCR_EL1 contain a
> mixture of UNKNOWN fields and RES0 bits.  For these,
> reset_unknown() does not work at present, since it sets all bits to
> junk values instead of just the wanted bits.
> 
> There is no need to craft another special helper just for that,
> since reset_unknown() almost does the appropriate thing anyway.
> This patch takes advantage of the ununused val field in struct
> sys_reg_desc to specify a mask of bits that should be initialised
> to zero instead of junk.
> 
> All existing users of reset_unknown() do not (and should not)
> define a value for val, so they will implicitly set it to zero,
> resulting in all bits being made UNKNOWN by this function: thus,
> this patch makes no functional change for currently defined
> registers.
> 
> Future patches will make use of non-zero val.
> 
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> ---
>  arch/arm64/kvm/sys_regs.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/sys_regs.h b/arch/arm64/kvm/sys_regs.h
> index cd710f8..24bac06 100644
> --- a/arch/arm64/kvm/sys_regs.h
> +++ b/arch/arm64/kvm/sys_regs.h
> @@ -89,7 +89,9 @@ static inline void reset_unknown(struct kvm_vcpu *vcpu,
>  {
>  	BUG_ON(!r->reg);
>  	BUG_ON(r->reg >= NR_SYS_REGS);
> -	__vcpu_sys_reg(vcpu, r->reg) = 0x1de7ec7edbadc0deULL;
> +
> +	/* If non-zero, r->val specifies which register bits are RES0: */
> +	__vcpu_sys_reg(vcpu, r->reg) = 0x1de7ec7edbadc0deULL & ~r->val;

nit: it would be nice to document this feature on the val field in the
sys_reg_desc structure above as well.


>  }
>  
>  static inline void reset_val(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
> -- 
> 2.1.4
> 

Thanks,

    Christoffer