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Date: Fri, 14 Jun 2024 16:23:18 +0100
From: Mark Rutland <mark.rutland@....com>
To: Anshuman Khandual <anshuman.khandual@....com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
will@...nel.org, catalin.marinas@....com,
Mark Brown <broonie@...nel.org>, James Clark <james.clark@....com>,
Rob Herring <robh@...nel.org>, Marc Zyngier <maz@...nel.org>,
Suzuki Poulose <suzuki.poulose@....com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
linux-perf-users@...r.kernel.org,
Oliver Upton <oliver.upton@...ux.dev>,
James Morse <james.morse@....com>, kvmarm@...ts.linux.dev
Subject: Re: [PATCH V18 6/9] KVM: arm64: nvhe: Disable branch generation in
nVHE guests
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@...nel.org>
> Cc: Oliver Upton <oliver.upton@...ux.dev>
> Cc: James Morse <james.morse@....com>
> Cc: Suzuki K Poulose <suzuki.poulose@....com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: kvmarm@...ts.linux.dev
> Cc: linux-arm-kernel@...ts.infradead.org
> CC: linux-kernel@...r.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> ----
> 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
>
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