Re: [PATCH v5 5/7] arm64: KVM: Add interface to set guest value for TRFCR register

[James Clark <james.clark@arm.com>]

On 26/02/2024 09:52, Suzuki K Poulose wrote:
> On 23/02/2024 16:39, James Clark wrote:
>>
>>
>> On 23/02/2024 10:03, Suzuki K Poulose wrote:
>>> On 20/02/2024 10:09, James Clark wrote:
>>>> Add an interface for the Coresight driver to use to set the value of
>>>> the
>>>> TRFCR register for the guest. This register controls the exclude
>>>> settings for trace at different exception levels, and is used to honor
>>>> the exclude_host and exclude_guest parameters from the Perf session.
>>>> This will be used to later write TRFCR_EL1 on nVHE at guest switch. For
>>>> VHE, the host trace is controlled by TRFCR_EL2 and thus we can write to
>>>> the TRFCR_EL1 immediately. Because guest writes to the register are
>>>> trapped, the value will persist and can't be modified.
>>>>
>>>> Instead of adding a load of infrastructure to share the host's per-cpu
>>>> offsets with the hypervisor, just define the new storage as a NR_CPUS
>>>> array.
>>>>
>>>> Signed-off-by: James Clark <james.clark@arm.com>
>>>> ---
>>>>    arch/arm64/include/asm/kvm_host.h |  3 +++
>>>>    arch/arm64/kernel/image-vars.h    |  1 +
>>>>    arch/arm64/kvm/debug.c            | 26 ++++++++++++++++++++++++++
>>>>    3 files changed, 30 insertions(+)
>>>>
>>>> diff --git a/arch/arm64/include/asm/kvm_host.h
>>>> b/arch/arm64/include/asm/kvm_host.h
>>>> index 85b5477bd1b4..56b7f7eca195 100644
>>>> --- a/arch/arm64/include/asm/kvm_host.h
>>>> +++ b/arch/arm64/include/asm/kvm_host.h
>>>> @@ -509,6 +509,7 @@ struct kvm_host_psci_config {
>>>>        bool psci_0_1_cpu_off_implemented;
>>>>        bool psci_0_1_migrate_implemented;
>>>>    };
>>>> +extern u64 ____cacheline_aligned kvm_guest_trfcr[NR_CPUS];
>>>>      extern struct kvm_host_psci_config
>>>> kvm_nvhe_sym(kvm_host_psci_config);
>>>>    #define kvm_host_psci_config CHOOSE_NVHE_SYM(kvm_host_psci_config)
>>>> @@ -1174,6 +1175,7 @@ void kvm_arch_vcpu_put_debug_state_flags(struct
>>>> kvm_vcpu *vcpu);
>>>>    void kvm_set_pmu_events(u32 set, struct perf_event_attr *attr);
>>>>    void kvm_clr_pmu_events(u32 clr);
>>>>    bool kvm_set_pmuserenr(u64 val);
>>>> +void kvm_etm_set_guest_trfcr(u64 trfcr_guest);
>>>>    #else
>>>>    static inline void kvm_set_pmu_events(u32 set, struct
>>>> perf_event_attr *attr) {}
>>>>    static inline void kvm_clr_pmu_events(u32 clr) {}
>>>> @@ -1181,6 +1183,7 @@ static inline bool kvm_set_pmuserenr(u64 val)
>>>>    {
>>>>        return false;
>>>>    }
>>>> +static inline void kvm_etm_set_guest_trfcr(u64 trfcr_guest) {}
>>>>    #endif
>>>>      void kvm_vcpu_load_vhe(struct kvm_vcpu *vcpu);
>>>> diff --git a/arch/arm64/kernel/image-vars.h
>>>> b/arch/arm64/kernel/image-vars.h
>>>> index 5e4dc72ab1bd..a451e4f10804 100644
>>>> --- a/arch/arm64/kernel/image-vars.h
>>>> +++ b/arch/arm64/kernel/image-vars.h
>>>> @@ -59,6 +59,7 @@ KVM_NVHE_ALIAS(alt_cb_patch_nops);
>>>>      /* Global kernel state accessed by nVHE hyp code. */
>>>>    KVM_NVHE_ALIAS(kvm_vgic_global_state);
>>>> +KVM_NVHE_ALIAS(kvm_guest_trfcr);
>>>>      /* Kernel symbols used to call panic() from nVHE hyp code (via
>>>> ERET). */
>>>>    KVM_NVHE_ALIAS(nvhe_hyp_panic_handler);
>>>> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
>>>> index 49a13e72ddd2..c8d936ce6e2b 100644
>>>> --- a/arch/arm64/kvm/debug.c
>>>> +++ b/arch/arm64/kvm/debug.c
>>>> @@ -22,6 +22,7 @@
>>>>                    DBG_MDSCR_MDE)
>>>>      static DEFINE_PER_CPU(u64, mdcr_el2);
>>>> +u64 ____cacheline_aligned kvm_guest_trfcr[NR_CPUS];
>>>>      /*
>>>>     * save/restore_guest_debug_regs
>>>> @@ -359,3 +360,28 @@ void kvm_arch_vcpu_put_debug_state_flags(struct
>>>> kvm_vcpu *vcpu)
>>>>        vcpu_clear_flag(vcpu, DEBUG_STATE_SAVE_TRBE);
>>>>        vcpu_clear_flag(vcpu, DEBUG_STATE_SAVE_TRFCR);
>>>>    }
>>>> +
>>>> +/*
>>>> + * Interface for the Coresight driver to use to set the value of the
>>>> TRFCR
>>>> + * register for the guest. This register controls the exclude
>>>> settings for trace
>>>> + * at different exception levels, and is used to honor the
>>>> exclude_host and
>>>> + * exclude_guest parameters from the Perf session.
>>>> + *
>>>> + * This will be used to later write TRFCR_EL1 on nVHE at guest
>>>> switch. For VHE,
>>>> + * the host trace is controlled by TRFCR_EL2 and thus we can write to
>>>> the
>>>> + * TRFCR_EL1 immediately. Because guest writes to the register are
>>>> trapped, the
>>>> + * value will persist and can't be modified. For pKVM,
>>>> kvm_guest_trfcr can't
>>>> + * be read by the hypervisor, so don't bother writing it.
>>>> + */
>>>> +void kvm_etm_set_guest_trfcr(u64 trfcr_guest)
>>>> +{
>>>> +    if
>>>> (WARN_ON_ONCE(!cpuid_feature_extract_unsigned_field(read_sysreg(id_aa64dfr0_el1),
>>>> +                                   ID_AA64DFR0_EL1_TraceFilt_SHIFT)))
>>>> +        return;
>>>> +
>>>> +    if (has_vhe())
>>>> +        write_sysreg_s(trfcr_guest, SYS_TRFCR_EL12);
>>>> +    else if (!is_protected_kvm_enabled())
>>>> +        kvm_guest_trfcr[smp_processor_id()] = trfcr_guest;
>>>
>>> smp_processor_id() could sleep in some configurations ? Should we switch
>>> to raw_smp_processor_id() to be safer ?
>>>
>>
>> I don't think so, it's #defined to raw_smp_processor_id() anyway. Unless
>> DEBUG_PREEMPT is on, then it's still raw_smp_processor_id() but it
>> validates that preemption is disabled so the value isn't stale. >
>> We actually want that validation, so should leave it as
> 
> This is actually what we want. i.e., We don't want the coresight drivers
> to make this call from a non-premptible context. Relying on the
> DEBUG_PREEMPT for detecting such erroneous uses is not a good idea,
> production kernels may not do this and a comment doesn't go far.
> So, I think we need a :
> 
> WARN_ON(preemptible()) in here and use smp_processor_id()
> 

Ok yeah that's easy enough to add. I was worried it would be redundant
with DEBUG_PREEMPT but I suppose it doesn't really cost anything.

>> smp_processor_id(). I can add a comment saying that this function should
>> only be called with preemption disabled, but I wouldn't add any extra
>> validation. Every smp_processor_id() call is already checked when
>> DEBUG_PREEMPT is on and this one doesn't seem to be special in any way.
>>
>> I also checked that the warning isn't triggered with DEBUG_PREEMPT on,
>> and there are also a lot of other smp_processor_id() calls on similar
>> paths in the Coresight driver.
> 
> Here, we are setting something for the KVM to program and if we set the
> filters on the wrong Physical CPU, then we mess up the tracing.
> 
> Suzuki
> 
>>
>>> Otherwise looks good to me.
>>>
>>> Suzuki
>>>
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(kvm_etm_set_guest_trfcr);
>>>
>