Re: [RFC PATCH v3 08/29] KVM: arm64: Make ID_AA64MMFR0_EL1 writable

[Eric Auger <eauger@redhat.com>]

Hi Reiji,
On 11/17/21 7:43 AM, Reiji Watanabe wrote:
> This patch adds id_reg_info for ID_AA64MMFR0_EL1 to make it
> writable by userspace.
> 
> Since ID_AA64MMFR0_EL1 stage 2 granule size fields don't follow the
> standard ID scheme, we need a special handling to validate those fields.
> 
> Signed-off-by: Reiji Watanabe <reijiw@google.com>
> ---
>  arch/arm64/kvm/sys_regs.c | 118 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 118 insertions(+)
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 5812e39602fe..772e3d3067b2 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -519,6 +519,113 @@ static int validate_id_aa64isar1_el1(struct kvm_vcpu *vcpu,
>  	return 0;
>  }
>  
> +/*
> + * Check if the requested stage2 translation granule size indicated in
> + * @mmfr0 is also indicated in @mmfr0_lim.  This function assumes that
> + * the stage1 granule size indicated in @mmfr0 has been validated already.
> + */
> +static int aa64mmfr0_tgran2_check(int field, u64 mmfr0, u64 mmfr0_lim)
> +{
> +	s64 tgran2, lim_tgran2, rtgran1;
> +	int f1;
> +	bool is_signed = true;
> +
> +	tgran2 = cpuid_feature_extract_unsigned_field(mmfr0, field);
> +	lim_tgran2 = cpuid_feature_extract_unsigned_field(mmfr0_lim, field);
> +	if (tgran2 == lim_tgran2)
> +		return 0;
> +
> +	if (tgran2 && lim_tgran2)
> +		return (tgran2 > lim_tgran2) ? -E2BIG : 0;
> +
> +	/*
> +	 * Either tgran2 or lim_tgran2 is zero.
> +	 * Need stage1 granule size to validate tgran2.
> +	 */
> +	switch (field) {
> +	case ID_AA64MMFR0_TGRAN4_2_SHIFT:
> +		f1 = ID_AA64MMFR0_TGRAN4_SHIFT;
> +		break;
> +	case ID_AA64MMFR0_TGRAN64_2_SHIFT:
> +		f1 = ID_AA64MMFR0_TGRAN64_SHIFT;
> +		break;
> +	case ID_AA64MMFR0_TGRAN16_2_SHIFT:
> +		f1 = ID_AA64MMFR0_TGRAN16_SHIFT;
> +		is_signed = false;
> +		break;
> +	default:
> +		/* Should never happen */
> +		WARN_ONCE(1, "Unexpected stage2 granule field (%d)\n", field);
> +		return 0;
> +	}

sorry my previous message was sent while I haven't finished :-(

if I understand correctly you forbid setting a granule that is not set
in the lim. So I would first compute whether the granule is set, would
it be through the TGranX (if _2 == 0) or though TGranX_2 if this latter
is not not. Do those computations both on val and lim and eventually
check if gran_val > gran_lim. The current code looks overly complicated
but maybe I miss the actual reason.

Eric
> +
> +	/*
> +	 * If tgran2 == 0 (&& lim_tgran2 != 0), the requested stage2 granule
> +	 * size is indicated in the stage1 granule size field of @mmfr0.
> +	 * So, validate the stage1 granule size against the stage2 limit
> +	 * granule size.
> +	 * If lim_tgran2 == 0 (&& tgran2 != 0), the stage2 limit granule size
> +	 * is indicated in the stage1 granule size field of @mmfr0_lim.
> +	 * So, validate the requested stage2 granule size against the stage1
> +	 * limit granule size.
> +	 */
> +
> +	 /* Get the relevant stage1 granule size to validate tgran2 */
> +	if (tgran2 == 0)
> +		/* The requested stage1 granule size */
> +		rtgran1 = cpuid_feature_extract_field(mmfr0, f1, is_signed);
> +	else /* lim_tgran2 == 0 */
> +		/* The stage1 limit granule size */
> +		rtgran1 = cpuid_feature_extract_field(mmfr0_lim, f1, is_signed);
> +
> +	/*
> +	 * Adjust the value of rtgran1 to compare with stage2 granule size,
> +	 * which indicates: 1: Not supported, 2: Supported, etc.
> +	 */
> +	if (is_signed)
> +		/* For signed, -1: Not supported, 0: Supported, etc. */
> +		rtgran1 += 0x2;
> +	else
> +		/* For unsigned, 0: Not supported, 1: Supported, etc. */
> +		rtgran1 += 0x1;
> +
> +	if ((tgran2 == 0) && (rtgran1 > lim_tgran2))
> +		/*
> +		 * The requested stage1 granule size (== the requested stage2
> +		 * granule size) is larger than the stage2 limit granule size.
> +		 */
> +		return -E2BIG;
> +	else if ((lim_tgran2 == 0) && (tgran2 > rtgran1))
> +		/*
> +		 * The requested stage2 granule size is larger than the stage1
> +		 * limit granulze size (== the stage2 limit granule size).
> +		 */
> +		return -E2BIG;
> +
> +	return 0;
> +}
> +
> +static int validate_id_aa64mmfr0_el1(struct kvm_vcpu *vcpu,
> +				     const struct id_reg_info *id_reg, u64 val)
> +{
> +	u64 limit = id_reg->vcpu_limit_val;
> +	int ret;
> +
> +	ret = aa64mmfr0_tgran2_check(ID_AA64MMFR0_TGRAN4_2_SHIFT, val, limit);
> +	if (ret)
> +		return ret;
> +
> +	ret = aa64mmfr0_tgran2_check(ID_AA64MMFR0_TGRAN64_2_SHIFT, val, limit);
> +	if (ret)
> +		return ret;
> +
> +	ret = aa64mmfr0_tgran2_check(ID_AA64MMFR0_TGRAN16_2_SHIFT, val, limit);
> +	if (ret)
> +		return ret;
> +
> +	return 0;
> +}
> +
>  static void init_id_aa64pfr0_el1_info(struct id_reg_info *id_reg)
>  {
>  	u64 limit = id_reg->vcpu_limit_val;
> @@ -625,6 +732,16 @@ static struct id_reg_info id_aa64isar1_el1_info = {
>  	.get_reset_val = get_reset_id_aa64isar1_el1,
>  };
>  
> +static struct id_reg_info id_aa64mmfr0_el1_info = {
> +	.sys_reg = SYS_ID_AA64MMFR0_EL1,
> +	.ftr_check_types = S_FCT(ID_AA64MMFR0_TGRAN4_SHIFT, FCT_LOWER_SAFE) |
> +			   S_FCT(ID_AA64MMFR0_TGRAN64_SHIFT, FCT_LOWER_SAFE) |
> +			   U_FCT(ID_AA64MMFR0_TGRAN4_2_SHIFT, FCT_IGNORE) |
> +			   U_FCT(ID_AA64MMFR0_TGRAN64_2_SHIFT, FCT_IGNORE) |
> +			   U_FCT(ID_AA64MMFR0_TGRAN16_2_SHIFT, FCT_IGNORE),
> +	.validate = validate_id_aa64mmfr0_el1,
> +};
> +
>  /*
>   * An ID register that needs special handling to control the value for the
>   * guest must have its own id_reg_info in id_reg_info_table.
> @@ -638,6 +755,7 @@ static struct id_reg_info *id_reg_info_table[KVM_ARM_ID_REG_MAX_NUM] = {
>  	[IDREG_IDX(SYS_ID_AA64PFR1_EL1)] = &id_aa64pfr1_el1_info,
>  	[IDREG_IDX(SYS_ID_AA64ISAR0_EL1)] = &id_aa64isar0_el1_info,
>  	[IDREG_IDX(SYS_ID_AA64ISAR1_EL1)] = &id_aa64isar1_el1_info,
> +	[IDREG_IDX(SYS_ID_AA64MMFR0_EL1)] = &id_aa64mmfr0_el1_info,
>  };
>  
>  static int validate_id_reg(struct kvm_vcpu *vcpu,
>