Re: [PATCH v5 2/3] KVM: arm64: limit PMU version to PMUv3 for ARMv8.1

From: Mark Rutland <mark.rutland@arm.com>
To: Andrew Murray <andrew.murray@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Marc Zyngier <maz@kernel.org>, Will Deacon <will@kernel.org>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v5 2/3] KVM: arm64: limit PMU version to PMUv3 for ARMv8.1
Date: Fri, 28 Feb 2020 16:51:22 +0000	[thread overview]
Message-ID: <20200228165122.GL36089@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <1580125469-23887-3-git-send-email-andrew.murray@arm.com>

On Mon, Jan 27, 2020 at 11:44:28AM +0000, Andrew Murray wrote:
> We currently expose the PMU version of the host to the guest via
> emulation of the DFR0_EL1 and AA64DFR0_EL1 debug feature registers.
> However many of the features offered beyond PMUv3 for 8.1 are not
> supported in KVM. Examples of this include support for the PMMIR
> registers (added in PMUv3 for ARMv8.4) and 64-bit event counters
> added in (PMUv3 for ARMv8.5).
> 
> Let's trap the Debug Feature Registers in order to limit
> PMUVer/PerfMon in the Debug Feature Registers to PMUv3 for ARMv8.1
> to avoid unexpected behaviour.
> 
> Signed-off-by: Andrew Murray <andrew.murray@arm.com>
> ---
>  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 6e919fa..1009878 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_1		4
> +#define ID_AA64DFR0_EL1_PMUVER_8_1	4
> +
>  #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 9f21659..3f0f3cc 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 ARMv8.1 */
> +		val = cpuid_feature_cap_signed_field_width(val,
> +						ID_AA64DFR0_PMUVER_SHIFT,
> +						4, ID_AA64DFR0_EL1_PMUVER_8_1);
> +	} else if (id == SYS_ID_DFR0_EL1) {
> +		/* Limit guests to PMUv3 for ARMv8.1 */
> +		val = cpuid_feature_cap_signed_field_width(val,
> +						ID_DFR0_PERFMON_SHIFT,
> +						4, ID_DFR0_EL1_PMUVER_8_1);
> +

Unfortunately, ID_AA64DFR0_EL1.PMUVer and ID_DFR0_EL1.PerfMon don't
follow the usual ID scheme, and cannot be treated as signed fields.

Per ARM DDI 0487E.a, page D13-2825, they follow an alternative scheme
that is treated as unsigned, with 0xF additionally being treated as an
architected PMU version not being implemented. For KVM we'll want to
convert 0xF to 0x0.

I'll respin these patches accordingly.

Thanks,
Mark.
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