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

[Marc Zyngier <maz@kernel.org>]

On Fri, 20 Jan 2023 14:04:12 +0000,
Marc Zyngier <maz@kernel.org> wrote:
> 
> 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)?

Scratch that, we need:

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index c6cbfe6b854b..6f6a928c92ec 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), .reset = reset_val, .visibility = pmu_visibility
 
 /* Macro to expand the PMEVCNTRn_EL0 register */
 #define PMU_PMEVCNTR_EL0(n)						\

But otherwise, this should be enough.

	M.

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