Subject: Re: [PATCH v4 2/3] KVM: arm64: limit PMU version to ARMv8.4

From: Marc Zyngier <[email-masked]>
Date: Thu, 23 Jan 2020 17:04:36 +0000

On 2020-01-23 11:58, Andrew Murray wrote:
> ARMv8.5-PMU introduces 64-bit event counters, however KVM doesn't yet
> support this. Let's trap the Debug Feature Registers in order to limit
> PMUVer/PerfMon in the Debug Feature Registers to PMUv3 for ARMv8.4.
> 
> Signed-off-by: Andrew Murray <[email-masked]>
> Reviewed-by: Suzuki K Poulose <[email-masked]>
> ---
>  arch/arm64/include/asm/sysreg.h |  5 +++++
>  arch/arm64/kvm/sys_regs.c       | 11 +++++++++++
>  2 files changed, 16 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/sysreg.h 
> b/arch/arm64/include/asm/sysreg.h
> index 6e919fafb43d..d969df417f88 100644
> --- a/arch/arm64/include/asm/sysreg.h
> +++ b/arch/arm64/include/asm/sysreg.h
> @@ -672,6 +672,11 @@
>  #define ID_AA64DFR0_TRACEVER_SHIFT	4
>  #define ID_AA64DFR0_DEBUGVER_SHIFT	0
> 
> +#define ID_DFR0_PERFMON_SHIFT		24
> +
> +#define ID_DFR0_EL1_PMUVER_8_4		5
> +#define ID_AA64DFR0_EL1_PMUVER_8_4	5
> +
>  #define ID_ISAR5_RDM_SHIFT		24
>  #define ID_ISAR5_CRC32_SHIFT		16
>  #define ID_ISAR5_SHA2_SHIFT		12
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 9f2165937f7d..028c93a88a51 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1085,6 +1085,17 @@ static u64 read_id_reg(const struct kvm_vcpu 
> *vcpu,
>  			 (0xfUL << ID_AA64ISAR1_API_SHIFT) |
>  			 (0xfUL << ID_AA64ISAR1_GPA_SHIFT) |
>  			 (0xfUL << ID_AA64ISAR1_GPI_SHIFT));
> +	} else if (id == SYS_ID_AA64DFR0_EL1) {
> +		/* Limit guests to PMUv3 for ARMv8.4 */
> +		val = cpuid_feature_cap_signed_field_width(val,
> +						ID_AA64DFR0_PMUVER_SHIFT,
> +						4, ID_AA64DFR0_EL1_PMUVER_8_4);
> +	} else if (id == SYS_ID_DFR0_EL1) {
> +		/* Limit guests to PMUv3 for ARMv8.4 */
> +		val = cpuid_feature_cap_signed_field_width(val,
> +						ID_DFR0_PERFMON_SHIFT,
> +						4, ID_DFR0_EL1_PMUVER_8_4);

I dont think this is right. Even today, we don't have support for PMMIR,
so pretending we support v8.4 is not true (the guest will get an UNDEF).
I would really stick to ARMv8.1 support here.

You could post it as a fix, to be taken right away.

         M.
-- 
Jazz is not dead. It just smells funny...

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