Re: [PATCH v19 10/11] KVM: arm64: nvhe: Disable branch generation in nVHE guests

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 2/3/25 06:13, Rob Herring (Arm) wrote:
> From: Anshuman Khandual <anshuman.khandual@arm.com>
> 
> While BRBE can record branches within guests, the host recording
> branches in guests is not supported by perf. Therefore, BRBE needs to be
> disabled on guest entry and restored on exit.
> 
> For nVHE, this requires explicit handling for guests. Before
> entering a guest, save the BRBE state and disable the it. When
> returning to the host, restore the state.
> 
> For VHE, it is not necessary. We initialize
> BRBCR_EL1.{E1BRE,E0BRE}=={0,0} at boot time, and HCR_EL2.TGE==1 while
> running in the host. We configure BRBCR_EL2.{E2BRE,E0HBRE} to enable
> branch recording in the host. When entering the guest, we set
> HCR_EL2.TGE==0 which means BRBCR_EL1 is used instead of BRBCR_EL2.
> Consequently for VHE, BRBE recording is disabled at EL1 and EL0 when
> running a guest.
> 
> Should recording in guests (by the host) ever be desired, the perf ABI
> will need to be extended to distinguish guest addresses (struct
> perf_branch_entry.priv) for starters. BRBE records would also need to be
> invalidated on guest entry/exit as guest/host EL1 and EL0 records can't
> be distinguished.
> 
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Co-developed-by: Rob Herring (Arm) <robh@kernel.org>
> Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
> ---
> v19:
>  - Rework due to v6.14 debug flag changes
>  - Redo commit message
> ---
>  arch/arm64/include/asm/kvm_host.h  |  2 ++
>  arch/arm64/kvm/debug.c             |  4 ++++
>  arch/arm64/kvm/hyp/nvhe/debug-sr.c | 32 ++++++++++++++++++++++++++++++++
>  3 files changed, 38 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 7cfa024de4e3..4fc246a1ee6b 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -619,6 +619,7 @@ struct kvm_host_data {
>  #define KVM_HOST_DATA_FLAG_HOST_SME_ENABLED		3
>  #define KVM_HOST_DATA_FLAG_TRBE_ENABLED			4
>  #define KVM_HOST_DATA_FLAG_EL1_TRACING_CONFIGURED	5
> +#define KVM_HOST_DATA_FLAG_HAS_BRBE			6

Although there is some variation in these feature names above, but seems
like KVM_HOST_DATA_FLAG_HAS_BRBE is an appropriate one for BRBE handling.

>  	unsigned long flags;
>  
>  	struct kvm_cpu_context host_ctxt;
> @@ -662,6 +663,7 @@ struct kvm_host_data {
>  		u64 trfcr_el1;
>  		/* Values of trap registers for the host before guest entry. */
>  		u64 mdcr_el2;
> +		u64 brbcr_el1;
>  	} host_debug_state;
>  
>  	/* Guest trace filter value */
> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> index 0e4c805e7e89..bc6015108a68 100644
> --- a/arch/arm64/kvm/debug.c
> +++ b/arch/arm64/kvm/debug.c
> @@ -81,6 +81,10 @@ void kvm_init_host_debug_data(void)
>  	    !(read_sysreg_s(SYS_PMBIDR_EL1) & PMBIDR_EL1_P))
>  		host_data_set_flag(HAS_SPE);
>  
> +	/* Check if we have BRBE implemented and available at the host */
> +	if (cpuid_feature_extract_unsigned_field(dfr0, ID_AA64DFR0_EL1_BRBE_SHIFT))
> +		host_data_set_flag(HAS_BRBE);
> +
>  	if (cpuid_feature_extract_unsigned_field(dfr0, ID_AA64DFR0_EL1_TraceFilt_SHIFT)) {
>  		/* Force disable trace in protected mode in case of no TRBE */
>  		if (is_protected_kvm_enabled())
> diff --git a/arch/arm64/kvm/hyp/nvhe/debug-sr.c b/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> index 2f4a4f5036bb..2a1c0f49792b 100644
> --- a/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> +++ b/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> @@ -92,12 +92,42 @@ static void __trace_switch_to_host(void)
>  			  *host_data_ptr(host_debug_state.trfcr_el1));
>  }
>  
> +static void __debug_save_brbe(u64 *brbcr_el1)
> +{
> +	*brbcr_el1 = 0;
> +
> +	/* Check if the BRBE is enabled */
> +	if (!(read_sysreg_el1(SYS_BRBCR) & (BRBCR_ELx_E0BRE | BRBCR_ELx_ExBRE)))
> +		return;
> +
> +	/*
> +	 * Prohibit branch record generation while we are in guest.
> +	 * Since access to BRBCR_EL1 is trapped, the guest can't
> +	 * modify the filtering set by the host.
> +	 */
> +	*brbcr_el1 = read_sysreg_el1(SYS_BRBCR);
> +	write_sysreg_el1(0, SYS_BRBCR);
> +}
> +
> +static void __debug_restore_brbe(u64 brbcr_el1)
> +{
> +	if (!brbcr_el1)
> +		return;
> +
> +	/* Restore BRBE controls */
> +	write_sysreg_el1(brbcr_el1, SYS_BRBCR);
> +}
> +
>  void __debug_save_host_buffers_nvhe(struct kvm_vcpu *vcpu)
>  {
>  	/* Disable and flush SPE data generation */
>  	if (host_data_test_flag(HAS_SPE))
>  		__debug_save_spe(host_data_ptr(host_debug_state.pmscr_el1));
>  
> +	/* Disable BRBE branch records */
> +	if (host_data_test_flag(HAS_BRBE))
> +		__debug_save_brbe(host_data_ptr(host_debug_state.brbcr_el1));
> +
>  	if (__trace_needs_switch())
>  		__trace_switch_to_guest();
>  }
> @@ -111,6 +141,8 @@ void __debug_restore_host_buffers_nvhe(struct kvm_vcpu *vcpu)
>  {
>  	if (host_data_test_flag(HAS_SPE))
>  		__debug_restore_spe(*host_data_ptr(host_debug_state.pmscr_el1));
> +	if (host_data_test_flag(HAS_BRBE))
> +		__debug_restore_brbe(*host_data_ptr(host_debug_state.brbcr_el1));
>  	if (__trace_needs_switch())
>  		__trace_switch_to_host();
>  }
> 

LGTM