Re: [RFC PATCH v3 03/29] KVM: arm64: Introduce struct id_reg_info

[Marc Zyngier <maz@kernel.org>]

On Wed, 17 Nov 2021 06:43:33 +0000,
Reiji Watanabe <reijiw@google.com> wrote:
> 
> This patch lays the groundwork to make ID registers writable.
> 
> Introduce struct id_reg_info for an ID register to manage the
> register specific control of its value for the guest, and provide set
> of functions commonly used for ID registers to make them writable.
> 
> The id_reg_info is used to do register specific initialization,
> validation of the ID register and etc.  Not all ID registers must
> have the id_reg_info. ID registers that don't have the id_reg_info
> are handled in a common way that is applied to all ID registers.
> 
> At present, changing an ID register from userspace is allowed only
> if the ID register has the id_reg_info, but that will be changed
> by the following patches.
> 
> No ID register has the structure yet and the following patches
> will add the id_reg_info for some ID registers.
> 
> Signed-off-by: Reiji Watanabe <reijiw@google.com>
> ---
>  arch/arm64/include/asm/sysreg.h |   1 +
>  arch/arm64/kvm/sys_regs.c       | 226 ++++++++++++++++++++++++++++++--
>  2 files changed, 218 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
> index 16b3f1a1d468..597609f26331 100644
> --- a/arch/arm64/include/asm/sysreg.h
> +++ b/arch/arm64/include/asm/sysreg.h
> @@ -1197,6 +1197,7 @@
>  #define ICH_VTR_TDS_MASK	(1 << ICH_VTR_TDS_SHIFT)
>  
>  #define ARM64_FEATURE_FIELD_BITS	4
> +#define ARM64_FEATURE_FIELD_MASK	((1ull << ARM64_FEATURE_FIELD_BITS) - 1)
>  
>  /* Create a mask for the feature bits of the specified feature. */
>  #define ARM64_FEATURE_MASK(x)	(GENMASK_ULL(x##_SHIFT + ARM64_FEATURE_FIELD_BITS - 1, x##_SHIFT))
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 5608d3410660..1552cd5581b7 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -265,6 +265,181 @@ static bool trap_raz_wi(struct kvm_vcpu *vcpu,
>  		return read_zero(vcpu, p);
>  }
>  
> +/*
> + * A value for FCT_LOWER_SAFE must be zero and changing that will affect
> + * ftr_check_types of id_reg_info.
> + */
> +enum feature_check_type {
> +	FCT_LOWER_SAFE = 0,
> +	FCT_HIGHER_SAFE,
> +	FCT_HIGHER_OR_ZERO_SAFE,
> +	FCT_EXACT,
> +	FCT_EXACT_OR_ZERO_SAFE,
> +	FCT_IGNORE,	/* Don't check (any value is fine) */
> +};
> +
> +static int arm64_check_feature_one(enum feature_check_type type, int val,
> +				   int limit)
> +{
> +	bool is_safe = false;
> +
> +	if (val == limit)
> +		return 0;
> +
> +	switch (type) {
> +	case FCT_LOWER_SAFE:
> +		is_safe = (val <= limit);
> +		break;
> +	case FCT_HIGHER_OR_ZERO_SAFE:
> +		if (val == 0) {
> +			is_safe = true;
> +			break;
> +		}
> +		fallthrough;
> +	case FCT_HIGHER_SAFE:
> +		is_safe = (val >= limit);
> +		break;
> +	case FCT_EXACT:
> +		break;
> +	case FCT_EXACT_OR_ZERO_SAFE:
> +		is_safe = (val == 0);
> +		break;
> +	case FCT_IGNORE:
> +		is_safe = true;
> +		break;
> +	default:
> +		WARN_ONCE(1, "Unexpected feature_check_type (%d)\n", type);
> +		break;
> +	}
> +
> +	return is_safe ? 0 : -1;
> +}
> +
> +#define	FCT_TYPE_MASK		0x7
> +#define	FCT_TYPE_SHIFT		1
> +#define	FCT_SIGN_MASK		0x1
> +#define	FCT_SIGN_SHIFT		0
> +#define	FCT_TYPE(val)	((val >> FCT_TYPE_SHIFT) & FCT_TYPE_MASK)
> +#define	FCT_SIGN(val)	((val >> FCT_SIGN_SHIFT) & FCT_SIGN_MASK)
> +
> +#define	MAKE_FCT(shift, type, sign)				\
> +	((u64)((((type) & FCT_TYPE_MASK) << FCT_TYPE_SHIFT) |	\
> +	       (((sign) & FCT_SIGN_MASK) << FCT_SIGN_SHIFT)) << (shift))
> +
> +/* For signed field */
> +#define	S_FCT(shift, type)	MAKE_FCT(shift, type, 1)
> +/* For unigned field */
> +#define	U_FCT(shift, type)	MAKE_FCT(shift, type, 0)
> +
> +/*
> + * @val and @lim are both a value of the ID register. The function checks
> + * if all features indicated in @val can be supported for guests on the host,
> + * which supports features indicated in @lim. @check_types indicates how
> + * features in the ID register needs to be checked.
> + * See comments for id_reg_info's ftr_check_types field for more detail.
> + */
> +static int arm64_check_features(u64 check_types, u64 val, u64 lim)
> +{
> +	int i;
> +
> +	for (i = 0; i < 64; i += ARM64_FEATURE_FIELD_BITS) {
> +		u8 ftr_check = (check_types >> i) & ARM64_FEATURE_FIELD_MASK;
> +		bool is_sign = FCT_SIGN(ftr_check);
> +		enum feature_check_type fctype = FCT_TYPE(ftr_check);
> +		int fval, flim, ret;
> +
> +		fval = cpuid_feature_extract_field(val, i, is_sign);
> +		flim = cpuid_feature_extract_field(lim, i, is_sign);
> +
> +		ret = arm64_check_feature_one(fctype, fval, flim);
> +		if (ret)
> +			return -E2BIG;
> +	}
> +	return 0;
> +}

All this logic seems to reinvent what we already have in
arch/arm64/kernel/cpufeature.c. I'd rather we rely on it and maintain
a single idreg handling library.

Could you outline what is missing in the cpufeature code that requires
you to invent your own? I'm sure Suzuki could help here to make it
directly usable.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.