Re: [PATCH 2/2] KVM: arm64: Use get_raz_reg() for userspace reads of PMSWINC_EL0

[Andrew Jones <drjones@redhat.com>]

On Mon, Sep 27, 2021 at 01:49:11PM +0100, Alexandru Elisei wrote:
> PMSWINC_EL0 is a write-only register and was initially part of the VCPU
> register state, but was later removed in commit 7a3ba3095a32 ("KVM:
> arm64: Remove PMSWINC_EL0 shadow register"). To prevent regressions, the
> register was kept accessible from userspace as Read-As-Zero (RAZ).
> 
> The read function that is used to handle userspace reads of this
> register is get_raz_id_reg(), which, while technically correct, as it
> returns 0, it is not semantically correct, as PMSWINC_EL0 is not an ID
> register as the function name suggests.
> 
> Add a new function, get_raz_reg(), to use it as the accessor for
> PMSWINC_EL0, as to not conflate get_raz_id_reg() to handle other types
> of registers.
> 
> No functional change intended.
> 
> Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> ---
>  arch/arm64/kvm/sys_regs.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 4adda8bf3168..1be827740f87 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1285,6 +1285,15 @@ static int set_raz_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
>  	return __set_id_reg(vcpu, rd, uaddr, true);
>  }
>  
> +static int get_raz_reg(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);
> +	const u64 val = 0;
> +
> +	return reg_to_user(uaddr, &val, id);
> +}
> +
>  static int set_wi_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
>  		      const struct kvm_one_reg *reg, void __user *uaddr)
>  {
> @@ -1647,7 +1656,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>  	 * previously (and pointlessly) advertised in the past...
>  	 */
>  	{ PMU_SYS_REG(SYS_PMSWINC_EL0),
> -	  .get_user = get_raz_id_reg, .set_user = set_wi_reg,
> +	  .get_user = get_raz_reg, .set_user = set_wi_reg,
>  	  .access = access_pmswinc, .reset = NULL },
>  	{ PMU_SYS_REG(SYS_PMSELR_EL0),
>  	  .access = access_pmselr, .reset = reset_pmselr, .reg = PMSELR_EL0 },
> -- 
> 2.33.0
>

What about replacing get_raz_id_reg() with this new function? Do really need
both?

Thanks,
drew


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