Re: [PATCH v4 5/6] KVM: arm64: Introduce ID register specific descriptor

[Marc Zyngier <maz@kernel.org>]

On Fri, 17 Mar 2023 05:06:36 +0000,
Jing Zhang <jingzhangos@google.com> wrote:
> 
> Introduce an ID feature register specific descriptor to include ID
> register specific fields and callbacks besides its corresponding
> general system register descriptor.
> New fields for ID register descriptor would be added later when it
> is necessary to support a writable ID register.

What would these be? Could they make sense for "normal" sysregs as
well?

> 
> No functional change intended.
> 
> Co-developed-by: Reiji Watanabe <reijiw@google.com>
> Signed-off-by: Reiji Watanabe <reijiw@google.com>
> Signed-off-by: Jing Zhang <jingzhangos@google.com>
> ---
>  arch/arm64/kvm/id_regs.c  | 187 +++++++++++++++++++++++++++-----------
>  arch/arm64/kvm/sys_regs.c |   2 +-
>  arch/arm64/kvm/sys_regs.h |   1 +
>  3 files changed, 138 insertions(+), 52 deletions(-)
> 
> diff --git a/arch/arm64/kvm/id_regs.c b/arch/arm64/kvm/id_regs.c
> index 3a87a3d2390d..9956c99d20f7 100644
> --- a/arch/arm64/kvm/id_regs.c
> +++ b/arch/arm64/kvm/id_regs.c
> @@ -18,6 +18,10 @@
>  
>  #include "sys_regs.h"
>  
> +struct id_reg_desc {
> +	const struct sys_reg_desc	reg_desc;
> +};
> +

What is the advantage in having this wrapping structure that forces us
to reinvent the wheel (the structure is different) over an additional
pointer or even a side table?

>  static u8 vcpu_pmuver(const struct kvm_vcpu *vcpu)
>  {
>  	if (kvm_vcpu_has_pmu(vcpu))
> @@ -334,21 +338,25 @@ static int set_id_dfr0_el1(struct kvm_vcpu *vcpu,
>  }
>  
>  /* sys_reg_desc initialiser for known cpufeature ID registers */
> -#define ID_SANITISED(name) {			\
> -	SYS_DESC(SYS_##name),			\
> -	.access	= access_id_reg,		\
> -	.get_user = get_id_reg,			\
> -	.set_user = set_id_reg,			\
> -	.visibility = id_visibility,		\
> +#define ID_SANITISED(name) {				\
> +	.reg_desc = {					\
> +		SYS_DESC(SYS_##name),			\
> +		.access	= access_id_reg,		\
> +		.get_user = get_id_reg,			\
> +		.set_user = set_id_reg,			\
> +		.visibility = id_visibility,		\
> +	},						\
>  }
>  
>  /* sys_reg_desc initialiser for known cpufeature ID registers */
> -#define AA32_ID_SANITISED(name) {		\
> -	SYS_DESC(SYS_##name),			\
> -	.access	= access_id_reg,		\
> -	.get_user = get_id_reg,			\
> -	.set_user = set_id_reg,			\
> -	.visibility = aa32_id_visibility,	\
> +#define AA32_ID_SANITISED(name) {			\
> +	.reg_desc = {					\
> +		SYS_DESC(SYS_##name),			\
> +		.access	= access_id_reg,		\
> +		.get_user = get_id_reg,			\
> +		.set_user = set_id_reg,			\
> +		.visibility = aa32_id_visibility,	\
> +	},						\
>  }
>  
>  /*
> @@ -356,12 +364,14 @@ static int set_id_dfr0_el1(struct kvm_vcpu *vcpu,
>   * register with encoding Op0=3, Op1=0, CRn=0, CRm=crm, Op2=op2
>   * (1 <= crm < 8, 0 <= Op2 < 8).
>   */
> -#define ID_UNALLOCATED(crm, op2) {			\
> -	Op0(3), Op1(0), CRn(0), CRm(crm), Op2(op2),	\
> -	.access = access_id_reg,			\
> -	.get_user = get_id_reg,				\
> -	.set_user = set_id_reg,				\
> -	.visibility = raz_visibility			\
> +#define ID_UNALLOCATED(crm, op2) {				\
> +	.reg_desc = {						\
> +		Op0(3), Op1(0), CRn(0), CRm(crm), Op2(op2),	\
> +		.access = access_id_reg,			\
> +		.get_user = get_id_reg,				\
> +		.set_user = set_id_reg,				\
> +		.visibility = raz_visibility			\
> +	},							\
>  }
>  
>  /*
> @@ -369,15 +379,17 @@ static int set_id_dfr0_el1(struct kvm_vcpu *vcpu,
>   * For now, these are exposed just like unallocated ID regs: they appear
>   * RAZ for the guest.
>   */
> -#define ID_HIDDEN(name) {			\
> -	SYS_DESC(SYS_##name),			\
> -	.access = access_id_reg,		\
> -	.get_user = get_id_reg,			\
> -	.set_user = set_id_reg,			\
> -	.visibility = raz_visibility,		\
> +#define ID_HIDDEN(name) {				\
> +	.reg_desc = {					\
> +		SYS_DESC(SYS_##name),			\
> +		.access = access_id_reg,		\
> +		.get_user = get_id_reg,			\
> +		.set_user = set_id_reg,			\
> +		.visibility = raz_visibility,		\
> +	},						\
>  }
>  
> -static const struct sys_reg_desc id_reg_descs[] = {
> +static const struct id_reg_desc id_reg_descs[KVM_ARM_ID_REG_NUM] = {
>  	/*
>  	 * ID regs: all ID_SANITISED() entries here must have corresponding
>  	 * entries in arm64_ftr_regs[].
> @@ -387,9 +399,13 @@ static const struct sys_reg_desc id_reg_descs[] = {
>  	/* CRm=1 */
>  	AA32_ID_SANITISED(ID_PFR0_EL1),
>  	AA32_ID_SANITISED(ID_PFR1_EL1),
> -	{ SYS_DESC(SYS_ID_DFR0_EL1), .access = access_id_reg,
> -	  .get_user = get_id_reg, .set_user = set_id_dfr0_el1,
> -	  .visibility = aa32_id_visibility, },
> +	{ .reg_desc = {
> +		SYS_DESC(SYS_ID_DFR0_EL1),
> +		.access = access_id_reg,
> +		.get_user = get_id_reg,
> +		.set_user = set_id_dfr0_el1,
> +		.visibility = aa32_id_visibility, },
> +	},
>  	ID_HIDDEN(ID_AFR0_EL1),
>  	AA32_ID_SANITISED(ID_MMFR0_EL1),
>  	AA32_ID_SANITISED(ID_MMFR1_EL1),
> @@ -418,8 +434,12 @@ static const struct sys_reg_desc id_reg_descs[] = {
>  
>  	/* AArch64 ID registers */
>  	/* CRm=4 */
> -	{ SYS_DESC(SYS_ID_AA64PFR0_EL1), .access = access_id_reg,
> -	  .get_user = get_id_reg, .set_user = set_id_aa64pfr0_el1, },
> +	{ .reg_desc = {
> +		SYS_DESC(SYS_ID_AA64PFR0_EL1),
> +		.access = access_id_reg,
> +		.get_user = get_id_reg,
> +		.set_user = set_id_aa64pfr0_el1, },
> +	},
>  	ID_SANITISED(ID_AA64PFR1_EL1),
>  	ID_UNALLOCATED(4, 2),
>  	ID_UNALLOCATED(4, 3),
> @@ -429,8 +449,12 @@ static const struct sys_reg_desc id_reg_descs[] = {
>  	ID_UNALLOCATED(4, 7),
>  
>  	/* CRm=5 */
> -	{ SYS_DESC(SYS_ID_AA64DFR0_EL1), .access = access_id_reg,
> -	  .get_user = get_id_reg, .set_user = set_id_aa64dfr0_el1, },
> +	{ .reg_desc = {
> +		SYS_DESC(SYS_ID_AA64DFR0_EL1),
> +		.access = access_id_reg,
> +		.get_user = get_id_reg,
> +		.set_user = set_id_aa64dfr0_el1, },
> +	},
>  	ID_SANITISED(ID_AA64DFR1_EL1),
>  	ID_UNALLOCATED(5, 2),
>  	ID_UNALLOCATED(5, 3),
> @@ -469,12 +493,12 @@ static const struct sys_reg_desc id_reg_descs[] = {
>   */
>  int emulate_id_reg(struct kvm_vcpu *vcpu, struct sys_reg_params *params)
>  {
> -	const struct sys_reg_desc *r;
> +	u32 id;
>  
> -	r = find_reg(params, id_reg_descs, ARRAY_SIZE(id_reg_descs));
> +	id = reg_to_encoding(params);
>  
> -	if (likely(r)) {
> -		perform_access(vcpu, params, r);
> +	if (likely(is_id_reg(id))) {
> +		perform_access(vcpu, params, &id_reg_descs[IDREG_IDX(id)].reg_desc);

How about minimising the diff and making the whole thing less verbose?

static const struct sys_reg_desc *id_to_id_reg_desc(struct sys_reg_params *params)
{
	u32 id;

	id = reg_to_encoding(params);
	if (is_id_reg(id))
		return &id_reg_descs[IDREG_IDX(id)].reg_desc;

	return NULL;
}

int emulate_id_reg(struct kvm_vcpu *vcpu, struct sys_reg_params *params)
{
	const struct sys_reg_desc *r;

	r = id_to_id_reg_desc(params);
	[...]
}

And use the helper everywhere?

>  	} else {
>  		print_sys_reg_msg(params,
>  				  "Unsupported guest id_reg access at: %lx [%08lx]\n",
> @@ -491,38 +515,102 @@ void kvm_arm_reset_id_regs(struct kvm_vcpu *vcpu)
>  	unsigned long i;
>  
>  	for (i = 0; i < ARRAY_SIZE(id_reg_descs); i++)
> -		if (id_reg_descs[i].reset)
> -			id_reg_descs[i].reset(vcpu, &id_reg_descs[i]);
> +		if (id_reg_descs[i].reg_desc.reset)
> +			id_reg_descs[i].reg_desc.reset(vcpu, &id_reg_descs[i].reg_desc);
>  }
>  
>  int kvm_arm_get_id_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
>  {
> -	return kvm_sys_reg_get_user(vcpu, reg,
> -				    id_reg_descs, ARRAY_SIZE(id_reg_descs));
> +	u64 __user *uaddr = (u64 __user *)(unsigned long)reg->addr;
> +	const struct sys_reg_desc *r;
> +	struct sys_reg_params params;
> +	u64 val;
> +	int ret;
> +	u32 id;
> +
> +	if (!index_to_params(reg->id, &params))
> +		return -ENOENT;
> +	id = reg_to_encoding(&params);
> +
> +	if (!is_id_reg(id))
> +		return -ENOENT;
> +
> +	r = &id_reg_descs[IDREG_IDX(id)].reg_desc;
> +	if (r->get_user) {
> +		ret = (r->get_user)(vcpu, r, &val);
> +	} else {
> +		ret = 0;
> +		val = vcpu->kvm->arch.id_regs[IDREG_IDX(id)];
> +	}
> +
> +	if (!ret)
> +		ret = put_user(val, uaddr);

How about the visibility? Why isn't it checked?

> +
> +	return ret;
>  }
>  
>  int kvm_arm_set_id_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
>  {
> -	return kvm_sys_reg_set_user(vcpu, reg,
> -				    id_reg_descs, ARRAY_SIZE(id_reg_descs));
> +	u64 __user *uaddr = (u64 __user *)(unsigned long)reg->addr;
> +	const struct sys_reg_desc *r;
> +	struct sys_reg_params params;
> +	u64 val;
> +	int ret;
> +	u32 id;
> +
> +	if (!index_to_params(reg->id, &params))
> +		return -ENOENT;
> +	id = reg_to_encoding(&params);
> +
> +	if (!is_id_reg(id))
> +		return -ENOENT;
> +
> +	if (get_user(val, uaddr))
> +		return -EFAULT;
> +
> +	r = &id_reg_descs[IDREG_IDX(id)].reg_desc;
> +
> +	if (sysreg_user_write_ignore(vcpu, r))
> +		return 0;
> +
> +	if (r->set_user) {
> +		ret = (r->set_user)(vcpu, r, val);
> +	} else {
> +		WARN_ONCE(1, "ID register set_user callback is NULL\n");

Why the shouting? We didn't do that before. What's changed?

> +		ret = 0;
> +	}
> +
> +	return ret;
>  }
>  
>  bool kvm_arm_check_idreg_table(void)
>  {
> -	return check_sysreg_table(id_reg_descs, ARRAY_SIZE(id_reg_descs), false);
> +	unsigned int i;
> +
> +	for (i = 0; i < ARRAY_SIZE(id_reg_descs); i++) {
> +		const struct sys_reg_desc *r = &id_reg_descs[i].reg_desc;
> +
> +		if (!is_id_reg(reg_to_encoding(r))) {
> +			kvm_err("id_reg table %pS entry %d not set correctly\n",
> +				&id_reg_descs[i].reg_desc, i);
> +			return false;
> +		}
> +	}
> +
> +	return true;
>  }
>  
>  int kvm_arm_walk_id_regs(struct kvm_vcpu *vcpu, u64 __user *uind)
>  {
> -	const struct sys_reg_desc *i2, *end2;
> +	const struct id_reg_desc *i2, *end2;
>  	unsigned int total = 0;
>  	int err;
>  
>  	i2 = id_reg_descs;
>  	end2 = id_reg_descs + ARRAY_SIZE(id_reg_descs);
>  
> -	while (i2 != end2) {
> -		err = walk_one_sys_reg(vcpu, i2++, &uind, &total);
> +	for (; i2 != end2; i2++) {
> +		err = walk_one_sys_reg(vcpu, &(i2->reg_desc), &uind, &total);
>  		if (err)
>  			return err;
>  	}
> @@ -540,12 +628,9 @@ void kvm_arm_set_default_id_regs(struct kvm *kvm)
>  	u64 val;
>  
>  	for (i = 0; i < ARRAY_SIZE(id_reg_descs); i++) {
> -		id = reg_to_encoding(&id_reg_descs[i]);
> -		if (WARN_ON_ONCE(!is_id_reg(id)))
> -			/* Shouldn't happen */
> -			continue;
> +		id = reg_to_encoding(&id_reg_descs[i].reg_desc);

Why have you dropped that check? If it shouldn't happen before, it
still shouldn't happen.

Thanks,

	M.

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