Re: [PATCH v2 1/8] KVM: arm64: PMU: Have reset_pmu_reg() to clear a register

[Marc Zyngier <maz@kernel.org>]

On Tue, 17 Jan 2023 01:35:35 +0000,
Reiji Watanabe <reijiw@google.com> wrote:
> 
> On vCPU reset, PMCNTEN{SET,CLR}_EL0, PMINTEN{SET,CLR}_EL1, and
> PMOVS{SET,CLR}_EL1 for a vCPU are reset by reset_pmu_reg().
> This function clears RAZ bits of those registers corresponding
> to unimplemented event counters on the vCPU, and sets bits
> corresponding to implemented event counters to a predefined
> pseudo UNKNOWN value (some bits are set to 1).
> 
> The function identifies (un)implemented event counters on the
> vCPU based on the PMCR_EL1.N value on the host. Using the host
> value for this would be problematic when KVM supports letting
> userspace set PMCR_EL1.N to a value different from the host value
> (some of the RAZ bits of those registers could end up being set to 1).
> 
> Fix reset_pmu_reg() to clear the registers so that it can ensure
> that all the RAZ bits are cleared even when the PMCR_EL1.N value
> for the vCPU is different from the host value.
> 
> Signed-off-by: Reiji Watanabe <reijiw@google.com>
> ---
>  arch/arm64/kvm/sys_regs.c | 10 +---------
>  1 file changed, 1 insertion(+), 9 deletions(-)
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index c6cbfe6b854b..ec4bdaf71a15 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -604,19 +604,11 @@ static unsigned int pmu_visibility(const struct kvm_vcpu *vcpu,
>  
>  static void reset_pmu_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
>  {
> -	u64 n, mask = BIT(ARMV8_PMU_CYCLE_IDX);
> -
>  	/* No PMU available, any PMU reg may UNDEF... */
>  	if (!kvm_arm_support_pmu_v3())
>  		return;

Is this still true? We remove the PMCR_EL0 access just below.

>  
> -	n = read_sysreg(pmcr_el0) >> ARMV8_PMU_PMCR_N_SHIFT;
> -	n &= ARMV8_PMU_PMCR_N_MASK;
> -	if (n)
> -		mask |= GENMASK(n - 1, 0);
> -
> -	reset_unknown(vcpu, r);
> -	__vcpu_sys_reg(vcpu, r->reg) &= mask;
> +	__vcpu_sys_reg(vcpu, r->reg) = 0;
>  }

At the end of the day, this function has no dependency on the host at
all, and only writes 0 to the per-vcpu register.

So why not get rid of it altogether and have:

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index c6cbfe6b854b..1d1514b89d75 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -976,7 +976,7 @@ static bool access_pmuserenr(struct kvm_vcpu *vcpu, struct sys_reg_params *p,
 	  trap_wcr, reset_wcr, 0, 0,  get_wcr, set_wcr }
 
 #define PMU_SYS_REG(r)						\
-	SYS_DESC(r), .reset = reset_pmu_reg, .visibility = pmu_visibility
+	SYS_DESC(r), .visibility = pmu_visibility
 
 /* Macro to expand the PMEVCNTRn_EL0 register */
 #define PMU_PMEVCNTR_EL0(n)						\

which would fall-back the specified reset value (zero by default)?

Thanks,

	M.

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