Re: [PATCH v5 08/12] KVM: arm64: PMU: Allow userspace to limit PMCR_EL0.N for the guest

[Shaoqin Huang <shahuang@redhat.com>]

On 8/26/23 06:34, Raghavendra Rao Ananta wrote:
> On Thu, Aug 24, 2023 at 1:50 AM Shaoqin Huang <shahuang@redhat.com> wrote:
>>
>>
>>
>> On 8/24/23 00:06, Raghavendra Rao Ananta wrote:
>>> On Tue, Aug 22, 2023 at 3:06 AM Shaoqin Huang <shahuang@redhat.com> wrote:
>>>>
>>>> Hi Raghavendra,
>>>>
>>>> On 8/17/23 08:30, Raghavendra Rao Ananta wrote:
>>>>> From: Reiji Watanabe <reijiw@google.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@google.com>
>>>>> Signed-off-by: Raghavendra Rao Ananta <rananta@google.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?
>>
> I believe you are referring to the guest trap access part of it
> (access_pmcr()). This patch focuses on the userspace access of PMCR
> via the KVM_SET_ONE_REG ioctl. Before this patch, KVM did not control
> the writes to the reg and userspace was free to write to any bits at
> any time.
> 

Oh yeah. Thanks for your explanation. My head sometimes broken down.

Thanks,
Shaoqin

> Thank you.
> Raghavendra
>> 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
>>
> 

-- 
Shaoqin