Subject: Re: [PATCH v8 2/6] KVM: arm64: Save ID registers' sanitized value per guest

From: Marc Zyngier <[email-masked]>
Date: Wed, 17 May 2023 08:41:14 +0100

On Wed, 03 May 2023 18:16:14 +0100,
Jing Zhang <[email-masked]> wrote:
> 
> Introduce id_regs[] in kvm_arch as a storage of guest's ID registers,
> and save ID registers' sanitized value in the array at KVM_CREATE_VM.
> Use the saved ones when ID registers are read by the guest or
> userspace (via KVM_GET_ONE_REG).
> 
> No functional change intended.
> 
> Co-developed-by: Reiji Watanabe <[email-masked]>
> Signed-off-by: Reiji Watanabe <[email-masked]>
> Signed-off-by: Jing Zhang <[email-masked]>
> ---
>  arch/arm64/include/asm/kvm_host.h | 20 ++++++++++++++
>  arch/arm64/kvm/arm.c              |  1 +
>  arch/arm64/kvm/id_regs.c          | 46 +++++++++++++++++++++++++------
>  arch/arm64/kvm/sys_regs.c         | 11 +++++++-
>  arch/arm64/kvm/sys_regs.h         |  3 +-
>  5 files changed, 69 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index bcd774d74f34..a7d4d9e093e3 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -177,6 +177,21 @@ struct kvm_smccc_features {
>  	unsigned long vendor_hyp_bmap;
>  };
>  
> +/*
> + * Emulated CPU ID registers per VM
> + * (Op0, Op1, CRn, CRm, Op2) of the ID registers to be saved in it
> + * is (3, 0, 0, crm, op2), where 1<=crm<8, 0<=op2<8.
> + *
> + * These emulated idregs are VM-wide, but accessed from the context of a vCPU.
> + * Access to id regs are guarded by kvm_arch.config_lock.
> + */
> +#define KVM_ARM_ID_REG_NUM	56

You already have this as part of patch #1 in another include file, and
then move it here. Surely you can do that in one go. I'd also like it
to be defined in terms of encodings, and not as a raw value.

> +#define IDREG_IDX(id)		(((sys_reg_CRm(id) - 1) << 3) | sys_reg_Op2(id))
> +#define IDREG(kvm, id)		kvm->arch.idregs.regs[IDREG_IDX(id)]

Missing brackets around 'kvm'.

> +struct kvm_idregs {
> +	u64 regs[KVM_ARM_ID_REG_NUM];
> +};
> +
>  typedef unsigned int pkvm_handle_t;
>  
>  struct kvm_protected_vm {
> @@ -243,6 +258,9 @@ struct kvm_arch {
>  	/* Hypercall features firmware registers' descriptor */
>  	struct kvm_smccc_features smccc_feat;
>  
> +	/* Emulated CPU ID registers */
> +	struct kvm_idregs idregs;
> +
>  	/*
>  	 * For an untrusted host VM, 'pkvm.handle' is used to lookup
>  	 * the associated pKVM instance in the hypervisor.
> @@ -1008,6 +1026,8 @@ int kvm_arm_vcpu_arch_has_attr(struct kvm_vcpu *vcpu,
>  long kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
>  				struct kvm_arm_copy_mte_tags *copy_tags);
>  
> +void kvm_arm_init_id_regs(struct kvm *kvm);
> +
>  /* Guest/host FPSIMD coordination helpers */
>  int kvm_arch_vcpu_run_map_fp(struct kvm_vcpu *vcpu);
>  void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu);
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 4b2e16e696a8..e34744c36406 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -153,6 +153,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>  
>  	set_default_spectre(kvm);
>  	kvm_arm_init_hypercalls(kvm);
> +	kvm_arm_init_id_regs(kvm);
>  
>  	/*
>  	 * Initialise the default PMUver before there is a chance to
> diff --git a/arch/arm64/kvm/id_regs.c b/arch/arm64/kvm/id_regs.c
> index 96b4c43a5100..e769223bcee2 100644
> --- a/arch/arm64/kvm/id_regs.c
> +++ b/arch/arm64/kvm/id_regs.c
> @@ -52,16 +52,9 @@ static u8 pmuver_to_perfmon(u8 pmuver)
>  	}
>  }
>  
> -/* Read a sanitised cpufeature ID register by sys_reg_desc */

Why getting rid of this comment instead of moving it next to the
(re-implemented) function?

> -static u64 read_id_reg(const struct kvm_vcpu *vcpu, struct sys_reg_desc const *r)
> +u64 kvm_arm_read_id_reg(const struct kvm_vcpu *vcpu, u32 id)
>  {
> -	u32 id = reg_to_encoding(r);
> -	u64 val;
> -
> -	if (sysreg_visible_as_raz(vcpu, r))
> -		return 0;
> -
> -	val = read_sanitised_ftr_reg(id);
> +	u64 val = IDREG(vcpu->kvm, id);
>  
>  	switch (id) {
>  	case SYS_ID_AA64PFR0_EL1:
> @@ -126,6 +119,14 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu, struct sys_reg_desc const *r
>  	return val;
>  }
>  
> +static u64 read_id_reg(const struct kvm_vcpu *vcpu, struct sys_reg_desc const *r)
> +{
> +	if (sysreg_visible_as_raz(vcpu, r))
> +		return 0;
> +
> +	return kvm_arm_read_id_reg(vcpu, reg_to_encoding(r));
> +}
> +
>  /* cpufeature ID register access trap handlers */
>  
>  static bool access_id_reg(struct kvm_vcpu *vcpu,
> @@ -458,3 +459,30 @@ int emulate_id_reg(struct kvm_vcpu *vcpu, struct sys_reg_params *params)
>  
>  	return 1;
>  }
> +
> +/*
> + * Set the guest's ID registers that are defined in id_reg_descs[]
> + * with ID_SANITISED() to the host's sanitized value.
> + */
> +void kvm_arm_init_id_regs(struct kvm *kvm)
> +{
> +	u64 val;
> +	u32 id;
> +	int i;
> +
> +	for (i = 0; i < ARRAY_SIZE(id_reg_descs); i++) {
> +		id = reg_to_encoding(&id_reg_descs[i]);
> +
> +		/*
> +		 * Some hidden ID registers which are not in arm64_ftr_regs[]
> +		 * would cause warnings from read_sanitised_ftr_reg().
> +		 * Skip those ID registers to avoid the warnings.
> +		 */
> +		if (id_reg_descs[i].visibility == raz_visibility)
> +			/* Hidden or reserved ID register */
> +			continue;

Are you sure? What about other visibility attributes that are normally
evaluated at runtime? This may work as a short term hack, but I'm not
sure this is the correct long-term solution...

	M.

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

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