Re: [V14 5/8] KVM: arm64: nvhe: Disable branch generation in nVHE guests

From: James Clark
Date: Tue Nov 14 2023 - 04:16:24 EST



On 14/11/2023 05:13, 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 is just to avoid capturing records
> in the guest kernel/userspace, which would be confusing the samples.
>
> 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 V14:
>
> - This is a new patch in the series
>
> arch/arm64/include/asm/kvm_host.h | 4 ++++
> arch/arm64/kvm/debug.c | 6 +++++
> arch/arm64/kvm/hyp/nvhe/debug-sr.c | 38 ++++++++++++++++++++++++++++++
> 3 files changed, 48 insertions(+)
>
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 68421c74283a..1faa0430d8dd 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -449,6 +449,8 @@ enum vcpu_sysreg {
> CNTHV_CVAL_EL2,
> PMSCR_EL1, /* Statistical profiling extension */
> TRFCR_EL1, /* Self-hosted trace filters */
> + BRBCR_EL1, /* Branch Record Buffer Control Register */
> + BRBFCR_EL1, /* Branch Record Buffer Function Control Register */
>
> NR_SYS_REGS /* Nothing after this line! */
> };
> @@ -753,6 +755,8 @@ struct kvm_vcpu_arch {
> #define VCPU_HYP_CONTEXT __vcpu_single_flag(iflags, BIT(7))
> /* Save trace filter controls */
> #define DEBUG_STATE_SAVE_TRFCR __vcpu_single_flag(iflags, BIT(8))
> +/* Save BRBE context if active */
> +#define DEBUG_STATE_SAVE_BRBE __vcpu_single_flag(iflags, BIT(9))
>
> /* 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 2ab41b954512..4055783c3d34 100644
> --- a/arch/arm64/kvm/debug.c
> +++ b/arch/arm64/kvm/debug.c
> @@ -354,6 +354,11 @@ void kvm_arch_vcpu_load_debug_state_flags(struct kvm_vcpu *vcpu)
> !(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) &&
> + (read_sysreg_s(SYS_BRBCR_EL1) & (BRBCR_ELx_E0BRE | BRBCR_ELx_ExBRE)))
> + vcpu_set_flag(vcpu, DEBUG_STATE_SAVE_BRBE);

Isn't this supposed to just be the feature check? Whether BRBE is
enabled or not is checked later in __debug_save_brbe() anyway.

It seems like it's possible to become enabled after this flag load part.

> }
>
> void kvm_arch_vcpu_put_debug_state_flags(struct kvm_vcpu *vcpu)
> @@ -361,6 +366,7 @@ 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_TRFCR);
> + vcpu_clear_flag(vcpu, DEBUG_STATE_SAVE_BRBE);
> }
>
> void kvm_etm_set_guest_trfcr(u64 trfcr_guest)
> diff --git a/arch/arm64/kvm/hyp/nvhe/debug-sr.c b/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> index 6174f710948e..e44a1f71a0f8 100644
> --- a/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> +++ b/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> @@ -93,6 +93,38 @@ static void __debug_restore_trace(struct kvm_cpu_context *host_ctxt,
> write_sysreg_s(ctxt_sys_reg(host_ctxt, TRFCR_EL1), SYS_TRFCR_EL1);
> }
>
> +static void __debug_save_brbe(struct kvm_cpu_context *host_ctxt)
> +{
> + ctxt_sys_reg(host_ctxt, BRBCR_EL1) = 0;
> + ctxt_sys_reg(host_ctxt, BRBFCR_EL1) = 0;
> +
> + /* Check if the BRBE is enabled */
> + if (!(ctxt_sys_reg(host_ctxt, BRBCR_EL1) & (BRBCR_ELx_E0BRE | BRBCR_ELx_ExBRE)))
> + return;

Doesn't this always fail, the host BRBCR_EL1 value was just cleared on
the line above.

Also, you need to read the register to determine if it was enabled or
not, so you might as well always store the real value, rather than 0 in
the not enabled case.

> +
> + /*
> + * Prohibit branch record generation while we are in guest.
> + * Since access to BRBCR_EL1 and BRBFCR_EL1 is trapped, the
> + * guest can't modify the filtering set by the host.
> + */
> + ctxt_sys_reg(host_ctxt, BRBCR_EL1) = read_sysreg_s(SYS_BRBCR_EL1);
> + ctxt_sys_reg(host_ctxt, BRBFCR_EL1) = read_sysreg_s(SYS_BRBFCR_EL1)
> + write_sysreg_s(0, SYS_BRBCR_EL1);
> + write_sysreg_s(0, SYS_BRBFCR_EL1);

Why does SYS_BRBFCR_EL1 need to be saved and restored? Only
BRBCR_ELx_E0BRE and BRBCR_ELx_ExBRE need to be cleared to disable BRBE.

> + isb();
> +}
> +
> +static void __debug_restore_brbe(struct kvm_cpu_context *host_ctxt)
> +{
> + if (!ctxt_sys_reg(host_ctxt, BRBCR_EL1) || !ctxt_sys_reg(host_ctxt, BRBFCR_EL1))
> + return;
> +
> + /* Restore BRBE controls */
> + write_sysreg_s(ctxt_sys_reg(host_ctxt, BRBCR_EL1), SYS_BRBCR_EL1);
> + write_sysreg_s(ctxt_sys_reg(host_ctxt, BRBFCR_EL1), SYS_BRBFCR_EL1);
> + isb();
> +}
> +
> void __debug_save_host_buffers_nvhe(struct kvm_cpu_context *host_ctxt,
> struct kvm_cpu_context *guest_ctxt)
> {
> @@ -102,6 +134,10 @@ void __debug_save_host_buffers_nvhe(struct kvm_cpu_context *host_ctxt,
>
> if (vcpu_get_flag(host_ctxt->__hyp_running_vcpu, DEBUG_STATE_SAVE_TRFCR))
> __debug_save_trace(host_ctxt, guest_ctxt);
> +
> + /* Disable BRBE branch records */
> + if (vcpu_get_flag(host_ctxt->__hyp_running_vcpu, DEBUG_STATE_SAVE_BRBE))
> + __debug_save_brbe(host_ctxt);
> }
>
> void __debug_switch_to_guest(struct kvm_vcpu *vcpu)
> @@ -116,6 +152,8 @@ void __debug_restore_host_buffers_nvhe(struct kvm_cpu_context *host_ctxt,
> __debug_restore_spe(host_ctxt);
> if (vcpu_get_flag(host_ctxt->__hyp_running_vcpu, DEBUG_STATE_SAVE_TRFCR))
> __debug_restore_trace(host_ctxt, guest_ctxt);
> + if (vcpu_get_flag(host_ctxt->__hyp_running_vcpu, DEBUG_STATE_SAVE_BRBE))
> + __debug_restore_brbe(host_ctxt);
> }
>
> void __debug_switch_to_host(struct kvm_vcpu *vcpu)