Re: [PATCH v2 6/7] KVM: arm64: Upgrade PMU support to ARMv8.4

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi Marc,

Had another look at the patch, comments below.

On 1/25/21 12:26 PM, Marc Zyngier wrote:
> Upgrading the PMU code from ARMv8.1 to ARMv8.4 turns out to be
> pretty easy. All that is required is support for PMMIR_EL1, which
> is read-only, and for which returning 0 is a valid option as long
> as we don't advertise STALL_SLOT as an implemented event.
>
> Let's just do that and adjust what we return to the guest.
>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/include/asm/sysreg.h |  3 +++
>  arch/arm64/kvm/pmu-emul.c       |  6 ++++++
>  arch/arm64/kvm/sys_regs.c       | 11 +++++++----
>  3 files changed, 16 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
> index 8b5e7e5c3cc8..2fb3f386588c 100644
> --- a/arch/arm64/include/asm/sysreg.h
> +++ b/arch/arm64/include/asm/sysreg.h
> @@ -846,7 +846,10 @@
>  
>  #define ID_DFR0_PERFMON_SHIFT		24
>  
> +#define ID_DFR0_PERFMON_8_0		0x3
>  #define ID_DFR0_PERFMON_8_1		0x4
> +#define ID_DFR0_PERFMON_8_4		0x5
> +#define ID_DFR0_PERFMON_8_5		0x6
>  
>  #define ID_ISAR4_SWP_FRAC_SHIFT		28
>  #define ID_ISAR4_PSR_M_SHIFT		24
> diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> index 398f6df1bbe4..72cd704a8368 100644
> --- a/arch/arm64/kvm/pmu-emul.c
> +++ b/arch/arm64/kvm/pmu-emul.c
> @@ -795,6 +795,12 @@ u64 kvm_pmu_get_pmceid(struct kvm_vcpu *vcpu, bool pmceid1)
>  		base = 0;
>  	} else {
>  		val = read_sysreg(pmceid1_el0);
> +		/*
> +		 * Don't advertise STALL_SLOT, as PMMIR_EL0 is handled
> +		 * as RAZ
> +		 */
> +		if (vcpu->kvm->arch.pmuver >= ID_AA64DFR0_PMUVER_8_4)
> +			val &= ~BIT_ULL(ARMV8_PMUV3_PERFCTR_STALL_SLOT - 32);

This is confusing the me. We have kvm->arch.pmuver set to the hardware PMU version
(as set by __armv8pmu_probe_pmu()), but we ignore it when reporting the PMU
version to the guest. Why do we do that? We limit the event number in
kvm_pmu_event_mask() based on the hardware PMU version, so even if we advertise
Armv8.4 PMU, support for all those extra events added by Arm8.1 PMU is missing (I
hope I understood the code correctly).

I looked at commit c854188ea010 ("KVM: arm64: limit PMU version to PMUv3 for
ARMv8.1") which changed read_id_reg() to report PMUv3 for Armv8.1 unconditionally,
and there's no explanation why PMUv3 for Armv8.1 was chosen instead of plain PMUv3
(PMUVer = 0b0100).

Thanks,

Alex

>  		base = 32;
>  	}
>  
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 8f79ec1fffa7..5da536ab738d 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1051,16 +1051,16 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu,
>  		/* Limit debug to ARMv8.0 */
>  		val &= ~FEATURE(ID_AA64DFR0_DEBUGVER);
>  		val |= FIELD_PREP(FEATURE(ID_AA64DFR0_DEBUGVER), 6);
> -		/* Limit guests to PMUv3 for ARMv8.1 */
> +		/* Limit guests to PMUv3 for ARMv8.4 */
>  		val = cpuid_feature_cap_perfmon_field(val,
>  						      ID_AA64DFR0_PMUVER_SHIFT,
> -						      kvm_vcpu_has_pmu(vcpu) ? ID_AA64DFR0_PMUVER_8_1 : 0);
> +						      kvm_vcpu_has_pmu(vcpu) ? ID_AA64DFR0_PMUVER_8_4 : 0);
>  		break;
>  	case SYS_ID_DFR0_EL1:
> -		/* Limit guests to PMUv3 for ARMv8.1 */
> +		/* Limit guests to PMUv3 for ARMv8.4 */
>  		val = cpuid_feature_cap_perfmon_field(val,
>  						      ID_DFR0_PERFMON_SHIFT,
> -						      kvm_vcpu_has_pmu(vcpu) ? ID_DFR0_PERFMON_8_1 : 0);
> +						      kvm_vcpu_has_pmu(vcpu) ? ID_DFR0_PERFMON_8_4 : 0);
>  		break;
>  	}
>  
> @@ -1496,6 +1496,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>  
>  	{ SYS_DESC(SYS_PMINTENSET_EL1), access_pminten, reset_unknown, PMINTENSET_EL1 },
>  	{ SYS_DESC(SYS_PMINTENCLR_EL1), access_pminten, reset_unknown, PMINTENSET_EL1 },
> +	{ SYS_DESC(SYS_PMMIR_EL1), trap_raz_wi },
>  
>  	{ SYS_DESC(SYS_MAIR_EL1), access_vm_reg, reset_unknown, MAIR_EL1 },
>  	{ SYS_DESC(SYS_AMAIR_EL1), access_vm_reg, reset_amair_el1, AMAIR_EL1 },
> @@ -1918,6 +1919,8 @@ static const struct sys_reg_desc cp15_regs[] = {
>  	{ Op1( 0), CRn( 9), CRm(14), Op2( 3), access_pmovs },
>  	{ AA32(HI), Op1( 0), CRn( 9), CRm(14), Op2( 4), access_pmceid },
>  	{ AA32(HI), Op1( 0), CRn( 9), CRm(14), Op2( 5), access_pmceid },
> +	/* PMMIR */
> +	{ Op1( 0), CRn( 9), CRm(14), Op2( 6), trap_raz_wi },
>  
>  	/* PRRR/MAIR0 */
>  	{ AA32(LO), Op1( 0), CRn(10), CRm( 2), Op2( 0), access_vm_reg, NULL, MAIR_EL1 },