Re: [PATCH v2 3/3] KVM: arm64: Remove AA64ZFR0_EL1 accessors

[Dave Martin <Dave.Martin@arm.com>]

On Mon, Nov 02, 2020 at 07:50:37PM +0100, Andrew Jones wrote:
> The AA64ZFR0_EL1 accessors are just the general accessors with
> its visibility function open-coded. It also skips the if-else
> chain in read_id_reg, but there's no reason not to go there.
> Indeed consolidating ID register accessors and removing lines
> of code make it worthwhile.
> 
> No functional change intended.

Nit: No statement of what the patch does.

> Signed-off-by: Andrew Jones <drjones@redhat.com>
> ---
>  arch/arm64/kvm/sys_regs.c | 61 +++++++--------------------------------
>  1 file changed, 11 insertions(+), 50 deletions(-)
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index b8822a20b1ea..e2d6fb83280e 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1156,6 +1156,16 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu,
>  static unsigned int id_visibility(const struct kvm_vcpu *vcpu,
>  				  const struct sys_reg_desc *r)
>  {
> +	u32 id = sys_reg((u32)r->Op0, (u32)r->Op1,
> +			 (u32)r->CRn, (u32)r->CRm, (u32)r->Op2);
> +
> +	switch (id) {
> +	case SYS_ID_AA64ZFR0_EL1:
> +		if (!vcpu_has_sve(vcpu))
> +			return REG_RAZ;
> +		break;
> +	}
> +

This should work, but I'm not sure it's preferable to giving affected
registers their own visibility check function.

Multiplexing all the ID regs through this one checker function will
introduce a bit of overhead for always-non-RAZ ID regs, but I'd guess
the impact is negligible given the other overheads on these paths.

>  	return 0;
>  }
>  
> @@ -1203,55 +1213,6 @@ static unsigned int sve_visibility(const struct kvm_vcpu *vcpu,
>  	return REG_HIDDEN_USER | REG_HIDDEN_GUEST;
>  }
>  
> -/* Generate the emulated ID_AA64ZFR0_EL1 value exposed to the guest */
> -static u64 guest_id_aa64zfr0_el1(const struct kvm_vcpu *vcpu)
> -{
> -	if (!vcpu_has_sve(vcpu))
> -		return 0;
> -
> -	return read_sanitised_ftr_reg(SYS_ID_AA64ZFR0_EL1);
> -}
> -
> -static bool access_id_aa64zfr0_el1(struct kvm_vcpu *vcpu,
> -				   struct sys_reg_params *p,
> -				   const struct sys_reg_desc *rd)
> -{
> -	if (p->is_write)
> -		return write_to_read_only(vcpu, p, rd);
> -
> -	p->regval = guest_id_aa64zfr0_el1(vcpu);
> -	return true;
> -}
> -
> -static int get_id_aa64zfr0_el1(struct kvm_vcpu *vcpu,
> -		const struct sys_reg_desc *rd,
> -		const struct kvm_one_reg *reg, void __user *uaddr)
> -{
> -	u64 val;
> -
> -	val = guest_id_aa64zfr0_el1(vcpu);
> -	return reg_to_user(uaddr, &val, reg->id);
> -}
> -
> -static int set_id_aa64zfr0_el1(struct kvm_vcpu *vcpu,
> -		const struct sys_reg_desc *rd,
> -		const struct kvm_one_reg *reg, void __user *uaddr)
> -{
> -	const u64 id = sys_reg_to_index(rd);
> -	int err;
> -	u64 val;
> -
> -	err = reg_from_user(&val, uaddr, id);
> -	if (err)
> -		return err;
> -
> -	/* This is what we mean by invariant: you can't change it. */
> -	if (val != guest_id_aa64zfr0_el1(vcpu))
> -		return -EINVAL;
> -
> -	return 0;
> -}
> -
>  /*
>   * cpufeature ID register user accessors
>   *
> @@ -1515,7 +1476,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>  	ID_SANITISED(ID_AA64PFR1_EL1),
>  	ID_UNALLOCATED(4,2),
>  	ID_UNALLOCATED(4,3),
> -	{ SYS_DESC(SYS_ID_AA64ZFR0_EL1), access_id_aa64zfr0_el1, .get_user = get_id_aa64zfr0_el1, .set_user = set_id_aa64zfr0_el1, },
> +	ID_SANITISED(ID_AA64ZFR0_EL1),

If keeping a dedicated helper, we could have a special macro for that, say

	ID_SANITISED_VISIBILITY(ID_AA64ZFR0_EL1, id_aa64zfr0_el1_visibility)

[...]

Cheers
---Dave
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