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Message-ID: <a0543866-1fac-6a3f-20cd-43b6d1263c0e@redhat.com>
Date: Thu, 24 Aug 2023 16:50:12 +0800
From: Shaoqin Huang <shahuang@...hat.com>
To: Raghavendra Rao Ananta <rananta@...gle.com>
Cc: Oliver Upton <oliver.upton@...ux.dev>,
Marc Zyngier <maz@...nel.org>,
Alexandru Elisei <alexandru.elisei@....com>,
James Morse <james.morse@....com>,
Suzuki K Poulose <suzuki.poulose@....com>,
Paolo Bonzini <pbonzini@...hat.com>,
Zenghui Yu <yuzenghui@...wei.com>,
Jing Zhang <jingzhangos@...gle.com>,
Reiji Watanabe <reijiw@...gle.com>,
Colton Lewis <coltonlewis@...gle.com>,
linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v5 08/12] KVM: arm64: PMU: Allow userspace to limit
PMCR_EL0.N for the guest
On 8/24/23 00:06, Raghavendra Rao Ananta wrote:
> On Tue, Aug 22, 2023 at 3:06 AM Shaoqin Huang <shahuang@...hat.com> wrote:
>>
>> Hi Raghavendra,
>>
>> On 8/17/23 08:30, Raghavendra Rao Ananta wrote:
>>> From: Reiji Watanabe <reijiw@...gle.com>
>>>
>>> KVM does not yet support userspace modifying PMCR_EL0.N (With
>>> the previous patch, KVM ignores what is written by upserspace).
>>> Add support userspace limiting PMCR_EL0.N.
>>>
>>> Disallow userspace to set PMCR_EL0.N to a value that is greater
>>> than the host value (KVM_SET_ONE_REG will fail), as KVM doesn't
>>> support more event counters than the host HW implements.
>>> Although this is an ABI change, this change only affects
>>> userspace setting PMCR_EL0.N to a larger value than the host.
>>> As accesses to unadvertised event counters indices is CONSTRAINED
>>> UNPREDICTABLE behavior, and PMCR_EL0.N was reset to the host value
>>> on every vCPU reset before this series, I can't think of any
>>> use case where a user space would do that.
>>>
>>> Also, ignore writes to read-only bits that are cleared on vCPU reset,
>>> and RES{0,1} bits (including writable bits that KVM doesn't support
>>> yet), as those bits shouldn't be modified (at least with
>>> the current KVM).
>>>
>>> Signed-off-by: Reiji Watanabe <reijiw@...gle.com>
>>> Signed-off-by: Raghavendra Rao Ananta <rananta@...gle.com>
>>> ---
>>> arch/arm64/include/asm/kvm_host.h | 3 ++
>>> arch/arm64/kvm/pmu-emul.c | 1 +
>>> arch/arm64/kvm/sys_regs.c | 49 +++++++++++++++++++++++++++++--
>>> 3 files changed, 51 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
>>> index 0f2dbbe8f6a7e..c15ec365283d1 100644
>>> --- a/arch/arm64/include/asm/kvm_host.h
>>> +++ b/arch/arm64/include/asm/kvm_host.h
>>> @@ -259,6 +259,9 @@ struct kvm_arch {
>>> /* PMCR_EL0.N value for the guest */
>>> u8 pmcr_n;
>>>
>>> + /* Limit value of PMCR_EL0.N for the guest */
>>> + u8 pmcr_n_limit;
>>> +
>>> /* Hypercall features firmware registers' descriptor */
>>> struct kvm_smccc_features smccc_feat;
>>> struct maple_tree smccc_filter;
>>> diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
>>> index ce7de6bbdc967..39ad56a71ad20 100644
>>> --- a/arch/arm64/kvm/pmu-emul.c
>>> +++ b/arch/arm64/kvm/pmu-emul.c
>>> @@ -896,6 +896,7 @@ int kvm_arm_set_vm_pmu(struct kvm *kvm, struct arm_pmu *arm_pmu)
>>> * while the latter does not.
>>> */
>>> kvm->arch.pmcr_n = arm_pmu->num_events - 1;
>>> + kvm->arch.pmcr_n_limit = arm_pmu->num_events - 1;
>>>
>>> return 0;
>>> }
>>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>>> index 2075901356c5b..c01d62afa7db4 100644
>>> --- a/arch/arm64/kvm/sys_regs.c
>>> +++ b/arch/arm64/kvm/sys_regs.c
>>> @@ -1086,6 +1086,51 @@ static int get_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
>>> return 0;
>>> }
>>>
>>> +static int set_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
>>> + u64 val)
>>> +{
>>> + struct kvm *kvm = vcpu->kvm;
>>> + u64 new_n, mutable_mask;
>>> + int ret = 0;
>>> +
>>> + new_n = FIELD_GET(ARMV8_PMU_PMCR_N, val);
>>> +
>>> + mutex_lock(&kvm->arch.config_lock);
>>> + if (unlikely(new_n != kvm->arch.pmcr_n)) {
>>> + /*
>>> + * The vCPU can't have more counters than the PMU
>>> + * hardware implements.
>>> + */
>>> + if (new_n <= kvm->arch.pmcr_n_limit)
>>> + kvm->arch.pmcr_n = new_n;
>>> + else
>>> + ret = -EINVAL;
>>> + }
>>> + mutex_unlock(&kvm->arch.config_lock);
>>
>> Another thing I am just wonder is that should we block any modification
>> to the pmcr_n after vm start to run? Like add one more checking
>> kvm_vm_has_ran_once() at the beginning of the set_pmcr() function.
>>
> Thanks for bringing it up. Reiji and I discussed about this. Checking
> for kvm_vm_has_ran_once() will be a good move, however, it will go
> against the ABI expectations of setting the PMCR. I'd like others to
> weigh in on this as well. What do you think?
>
> Thank you.
> Raghavendra
Before this change, kvm not allowed userspace to change the pmcr_n, but
allowed to change the lower ARMV8_PMU_PMCR_MASK bits. With this change,
we now allow to change the pmcr_n, we should not block the change to
ARMV8_PMU_PMCR_MASK after vm start to run, but how about we just block
the change to ARMV8_PMU_PMCR_N after vm start to run?
Thanks,
Shaoqin
>> Thanks,
>> Shaoqin
>>
>>> + if (ret)
>>> + return ret;
>>> +
>>> + /*
>>> + * Ignore writes to RES0 bits, read only bits that are cleared on
>>> + * vCPU reset, and writable bits that KVM doesn't support yet.
>>> + * (i.e. only PMCR.N and bits [7:0] are mutable from userspace)
>>> + * The LP bit is RES0 when FEAT_PMUv3p5 is not supported on the vCPU.
>>> + * But, we leave the bit as it is here, as the vCPU's PMUver might
>>> + * be changed later (NOTE: the bit will be cleared on first vCPU run
>>> + * if necessary).
>>> + */
>>> + mutable_mask = (ARMV8_PMU_PMCR_MASK | ARMV8_PMU_PMCR_N);
>>> + val &= mutable_mask;
>>> + val |= (__vcpu_sys_reg(vcpu, r->reg) & ~mutable_mask);
>>> +
>>> + /* The LC bit is RES1 when AArch32 is not supported */
>>> + if (!kvm_supports_32bit_el0())
>>> + val |= ARMV8_PMU_PMCR_LC;
>>> +
>>> + __vcpu_sys_reg(vcpu, r->reg) = val;
>>> + return 0;
>>> +}
>>> +
>>> /* Silly macro to expand the DBG{BCR,BVR,WVR,WCR}n_EL1 registers in one go */
>>> #define DBG_BCR_BVR_WCR_WVR_EL1(n) \
>>> { SYS_DESC(SYS_DBGBVRn_EL1(n)), \
>>> @@ -2147,8 +2192,8 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>>> { SYS_DESC(SYS_CTR_EL0), access_ctr },
>>> { SYS_DESC(SYS_SVCR), undef_access },
>>>
>>> - { PMU_SYS_REG(PMCR_EL0), .access = access_pmcr,
>>> - .reset = reset_pmcr, .reg = PMCR_EL0, .get_user = get_pmcr },
>>> + { PMU_SYS_REG(PMCR_EL0), .access = access_pmcr, .reset = reset_pmcr,
>>> + .reg = PMCR_EL0, .get_user = get_pmcr, .set_user = set_pmcr },
>>> { PMU_SYS_REG(PMCNTENSET_EL0),
>>> .access = access_pmcnten, .reg = PMCNTENSET_EL0 },
>>> { PMU_SYS_REG(PMCNTENCLR_EL0),
>>
>> --
>> Shaoqin
>>
>
--
Shaoqin
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