Re: [PATCH v1 3/5] arm64: KVM: Add interface to set guest value for TRFCR register

[Marc Zyngier]

On Thu, 28 Sep 2023 16:16:07 +0100,
James Clark <james.clark@xxxxxxx> 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, TRFCR_EL1 is written immediately. Because guest writes
> to the register are trapped, the value will persist and can't be
> modified.
> 
> The settings must be copied to the vCPU before each run in the same
> way that PMU events are because the per-cpu struct isn't accessible in
> protected mode.
> 
> Now that both guest and host values are saved, rename trfcr_el1 to
> host_trfcr_el1 to make it clear that's the value that should be restored
> on return to the host.
> 
> Reviewed-by: Mark Brown <broonie@xxxxxxxxxx> (sysreg)
> Signed-off-by: James Clark <james.clark@xxxxxxx>
> ---
>  arch/arm64/include/asm/kvm_host.h  |  6 +++++-
>  arch/arm64/kvm/arm.c               |  1 +
>  arch/arm64/kvm/debug.c             | 21 +++++++++++++++++++++
>  arch/arm64/kvm/hyp/nvhe/debug-sr.c |  4 ++--
>  arch/arm64/tools/sysreg            |  4 ++++
>  5 files changed, 33 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 498f922f4f41..0e57827a0cf2 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -547,7 +547,8 @@ struct kvm_vcpu_arch {
>  		/* Statistical profiling extension */
>  		u64 pmscr_el1;
>  		/* Self-hosted trace */
> -		u64 trfcr_el1;
> +		u64 host_trfcr_el1;
> +		u64 guest_trfcr_el1;
>  	} host_debug_state;

I think it is high time we stop having *guest* state in a structure
that is obviously for the host, starting by moving the breakpoint and
watchpoint out, and into the sysreg array.

And then TRFCR_EL1 can join the fun. But it is pretty unclear whether
that's actually the guest state.

>  
>  	/* VGIC state */
> @@ -1097,6 +1098,8 @@ 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);
> +void kvm_etm_update_vcpu_events(struct kvm_vcpu *vcpu);
>  #else
>  static inline void kvm_set_pmu_events(u32 set, struct perf_event_attr *attr) {}
>  static inline void kvm_clr_pmu_events(u32 clr) {}
> @@ -1104,6 +1107,7 @@ 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_sysregs_vhe(struct kvm_vcpu *vcpu);
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 1bfdd583b261..65e805dc1d7a 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -958,6 +958,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
>  		kvm_vgic_flush_hwstate(vcpu);
>  
>  		kvm_pmu_update_vcpu_events(vcpu);
> +		kvm_etm_update_vcpu_events(vcpu);
>  
>  		/*
>  		 * Ensure we set mode to IN_GUEST_MODE after we disable
> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> index 6a1bad1a921b..379d2677961f 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);
> +static DEFINE_PER_CPU(u64, guest_trfcr);

Hold on a sec. Why is that global? I'd expect the tracing to be
specific to a vcpu, and not to affect *everything*.

>  
>  /**
>   * save/restore_guest_debug_regs
> @@ -342,3 +343,23 @@ void kvm_arch_vcpu_put_debug_state_flags(struct kvm_vcpu *vcpu)
>  	vcpu_clear_flag(vcpu, DEBUG_STATE_SAVE_SPE);
>  	vcpu_clear_flag(vcpu, DEBUG_STATE_SAVE_TRFCR);
>  }
> +
> +void kvm_etm_set_guest_trfcr(u64 trfcr_guest)
> +{
> +	if (has_vhe())
> +		write_sysreg_s(trfcr_guest, SYS_TRFCR_EL12);
> +	else
> +		*this_cpu_ptr(&guest_trfcr) = trfcr_guest;
> +}
> +EXPORT_SYMBOL_GPL(kvm_etm_set_guest_trfcr);

In what context can this be called? What if we want to expose TRFCR to
guests instead? It looks to me that this ultimately precludes such
use.

> +
> +/*
> + * Updates the vcpu's view of the etm events for this cpu. Must be
> + * called before every vcpu run after disabling interrupts, to ensure
> + * that an interrupt cannot fire and update the structure.
> + */
> +void kvm_etm_update_vcpu_events(struct kvm_vcpu *vcpu)
> +{
> +	if (!has_vhe() && vcpu_get_flag(vcpu, DEBUG_STATE_SAVE_TRFCR))
> +		vcpu->arch.host_debug_state.guest_trfcr_el1 = *this_cpu_ptr(&guest_trfcr);

But what is the point of this per-vcpu field if all you care about is
some per-CPU data?

> +}
> diff --git a/arch/arm64/kvm/hyp/nvhe/debug-sr.c b/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> index 89c208112eb7..55bc01e9808f 100644
> --- a/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> +++ b/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> @@ -86,7 +86,7 @@ void __debug_save_host_buffers_nvhe(struct kvm_vcpu *vcpu)
>  		__debug_save_spe(&vcpu->arch.host_debug_state.pmscr_el1);
>  	/* Disable and flush Self-Hosted Trace generation */
>  	if (vcpu_get_flag(vcpu, DEBUG_STATE_SAVE_TRFCR))
> -		__debug_save_trace(&vcpu->arch.host_debug_state.trfcr_el1);
> +		__debug_save_trace(&vcpu->arch.host_debug_state.host_trfcr_el1);
>  }
`<>
>  void __debug_switch_to_guest(struct kvm_vcpu *vcpu)
> @@ -99,7 +99,7 @@ void __debug_restore_host_buffers_nvhe(struct kvm_vcpu *vcpu)
>  	if (vcpu_get_flag(vcpu, DEBUG_STATE_SAVE_SPE))
>  		__debug_restore_spe(vcpu->arch.host_debug_state.pmscr_el1);
>  	if (vcpu_get_flag(vcpu, DEBUG_STATE_SAVE_TRFCR))
> -		__debug_restore_trace(vcpu->arch.host_debug_state.trfcr_el1);
> +		__debug_restore_trace(vcpu->arch.host_debug_state.host_trfcr_el1);
>  }
>  
>  void __debug_switch_to_host(struct kvm_vcpu *vcpu)
> diff --git a/arch/arm64/tools/sysreg b/arch/arm64/tools/sysreg
> index 992722c0c23b..295a4a625b8b 100644
> --- a/arch/arm64/tools/sysreg
> +++ b/arch/arm64/tools/sysreg
> @@ -2661,3 +2661,7 @@ EndSysreg
>  Sysreg	TRFCR_EL2	3	4	1	2	1
>  Fields	TRFCR_EL2
>  EndSysreg
> +
> +Sysreg TRFCR_EL12	3	5	1	2	1
> +Fields	TRFCR_ELx
> +EndSysreg

Please move this to the first patch.

	M.

-- 
Without deviation from the norm, progress is not possible.