Re: [RFC v2 6/7] kvm: arm64: make ID registers configurable

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On Thu, Sep 17, 2020 at 08:01:00PM +0800, Peng Liang wrote:
> It's time to make ID registers configurable.  When userspace (but not
> guest) want to set the values of ID registers, save the value in
> sysreg file so that guest can read the modified values.
> 
> Signed-off-by: zhanghailiang <zhang.zhanghailiang@xxxxxxxxxx>
> Signed-off-by: Peng Liang <liangpeng10@xxxxxxxxxx>
> ---
>  arch/arm64/kvm/sys_regs.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index a642ecfebe0a..881b66494524 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1263,10 +1263,6 @@ static int set_id_aa64zfr0_el1(struct kvm_vcpu *vcpu,
>  
>  /*
>   * cpufeature ID register user accessors
> - *
> - * For now, these registers are immutable for userspace, so no values
> - * are stored, and for set_id_reg() we don't allow the effective value
> - * to be changed.
>   */
>  static int __get_id_reg(struct kvm_vcpu *vcpu,
>  			const struct sys_reg_desc *rd, void __user *uaddr,
> @@ -1290,9 +1286,15 @@ static int __set_id_reg(struct kvm_vcpu *vcpu,
>  	if (err)
>  		return err;
>  
> -	/* This is what we mean by invariant: you can't change it. */
> -	if (val != read_id_reg(vcpu, rd, raz))
> -		return -EINVAL;
> +	if (raz) {
> +		if (val != read_id_reg(vcpu, rd, raz))

val != 0 ?

> +			return -EINVAL;
> +	} else {
> +		u32 reg_id = sys_reg((u32)rd->Op0, (u32)rd->Op1, (u32)rd->CRn,
> +				     (u32)rd->CRm, (u32)rd->Op2);
> +		/* val should be checked in check_user */

It really doesn't make sense to share this trivial set_user function and
have different check functions. Just don't share the set_user function.

> +		__vcpu_sys_reg(vcpu, ID_REG_INDEX(reg_id)) = val;
> +	}
>  
>  	return 0;
>  }
> -- 
> 2.26.2
> 

Thanks,
drew

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