Re: [PATCH V18 6/9] KVM: arm64: nvhe: Disable branch generation in nVHE guests

[Mark Rutland]

On Thu, Jun 13, 2024 at 11:47:28AM +0530, Anshuman Khandual wrote:
> Disable the BRBE before we enter the guest, saving the status and enable it
> back once we get out of the guest. This avoids capturing branch records in
> the guest kernel or userspace, which would be confusing the host samples.

It'd be good to explain why we need to do this for nVHE, but not for
VHE. I *think* that you're relying on BRBCR_EL2.EL0HBRE being ignored
when HCR_EL2.TGE == 0, and BRBCR_EL1.E{1,0}BRE being initialized to 0
out-of-reset.

What should a user do if they *want* samples from a guest? Is that
possible to do on other architectures, or do is that always prevented?

Mark.

> 
> Cc: Marc Zyngier <maz@xxxxxxxxxx>
> Cc: Oliver Upton <oliver.upton@xxxxxxxxx>
> Cc: James Morse <james.morse@xxxxxxx>
> Cc: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: kvmarm@xxxxxxxxxxxxxxx
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> CC: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> ----
> Changes in V18:
> 
> - Used host_data_ptr() to access host_debug_state.brbcr_el1 register
> - Changed DEBUG_STATE_SAVE_BRBE to use BIT(7)
> - Reverted back iflags as u8
> 
>  arch/arm64/include/asm/kvm_host.h  |  3 +++
>  arch/arm64/kvm/debug.c             |  5 +++++
>  arch/arm64/kvm/hyp/nvhe/debug-sr.c | 31 ++++++++++++++++++++++++++++++
>  3 files changed, 39 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 36b8e97bf49e..db922c10bd2a 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -579,6 +579,7 @@ 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;
>  };
>  
> @@ -842,6 +843,8 @@ struct kvm_vcpu_arch {
>  #define DEBUG_STATE_SAVE_SPE	__vcpu_single_flag(iflags, BIT(5))
>  /* Save TRBE context if active  */
>  #define DEBUG_STATE_SAVE_TRBE	__vcpu_single_flag(iflags, BIT(6))
> +/* Save BRBE context if active  */
> +#define DEBUG_STATE_SAVE_BRBE	__vcpu_single_flag(iflags, BIT(7))
>  
>  /* SVE enabled for host EL0 */
>  #define HOST_SVE_ENABLED	__vcpu_single_flag(sflags, BIT(0))
> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> index ce8886122ed3..8fa648943f0f 100644
> --- a/arch/arm64/kvm/debug.c
> +++ b/arch/arm64/kvm/debug.c
> @@ -336,10 +336,15 @@ void kvm_arch_vcpu_load_debug_state_flags(struct kvm_vcpu *vcpu)
>  	if (cpuid_feature_extract_unsigned_field(dfr0, ID_AA64DFR0_EL1_TraceBuffer_SHIFT) &&
>  	    !(read_sysreg_s(SYS_TRBIDR_EL1) & TRBIDR_EL1_P))
>  		vcpu_set_flag(vcpu, DEBUG_STATE_SAVE_TRBE);
> +
> +	/* Check if we have BRBE implemented and available at the host */
> +	if (cpuid_feature_extract_unsigned_field(dfr0, ID_AA64DFR0_EL1_BRBE_SHIFT))
> +		vcpu_set_flag(vcpu, DEBUG_STATE_SAVE_BRBE);
>  }
>  
>  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_TRBE);
> +	vcpu_clear_flag(vcpu, DEBUG_STATE_SAVE_BRBE);
>  }
> diff --git a/arch/arm64/kvm/hyp/nvhe/debug-sr.c b/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> index 53efda0235cf..97e861df1b45 100644
> --- a/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> +++ b/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> @@ -79,6 +79,32 @@ static void __debug_restore_trace(u64 trfcr_el1)
>  	write_sysreg_el1(trfcr_el1, SYS_TRFCR);
>  }
>  
> +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 */
> @@ -87,6 +113,9 @@ void __debug_save_host_buffers_nvhe(struct kvm_vcpu *vcpu)
>  	/* Disable and flush Self-Hosted Trace generation */
>  	if (vcpu_get_flag(vcpu, DEBUG_STATE_SAVE_TRBE))
>  		__debug_save_trace(host_data_ptr(host_debug_state.trfcr_el1));
> +	/* Disable BRBE branch records */
> +	if (vcpu_get_flag(vcpu, DEBUG_STATE_SAVE_BRBE))
> +		__debug_save_brbe(host_data_ptr(host_debug_state.brbcr_el1));
>  }
>  
>  void __debug_switch_to_guest(struct kvm_vcpu *vcpu)
> @@ -100,6 +129,8 @@ void __debug_restore_host_buffers_nvhe(struct kvm_vcpu *vcpu)
>  		__debug_restore_spe(*host_data_ptr(host_debug_state.pmscr_el1));
>  	if (vcpu_get_flag(vcpu, DEBUG_STATE_SAVE_TRBE))
>  		__debug_restore_trace(*host_data_ptr(host_debug_state.trfcr_el1));
> +	if (vcpu_get_flag(vcpu, DEBUG_STATE_SAVE_BRBE))
> +		__debug_restore_brbe(*host_data_ptr(host_debug_state.brbcr_el1));
>  }
>  
>  void __debug_switch_to_host(struct kvm_vcpu *vcpu)
> -- 
> 2.25.1
>