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Message-ID: <20858eb9-a4d0-41be-ad1d-2a5f2d2fa0de@arm.com>
Date: Tue, 21 Nov 2023 16:42:18 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: James Clark <james.clark@....com>
Cc: Mark Brown <broonie@...nel.org>, 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,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
will@...nel.org, catalin.marinas@....com, mark.rutland@....com
Subject: Re: [V14 5/8] KVM: arm64: nvhe: Disable branch generation in nVHE
guests
On 11/14/23 14:46, James Clark wrote:
>
>
> 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@...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 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.
Okay, will make it just a feature check via ID_AA64DFR0_EL1_BRBE_SHIFT.
>
> It seems like it's possible to become enabled after this flag load part.
Agreed.
>
>> }
>>
>> 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.
Agreed, this error might have slipped in while converting to ctxt_sys_reg().
>
> Also, you need to read the register to determine if it was enabled or
Right
> not, so you might as well always store the real value, rather than 0 in
> the not enabled case.
But if it is not enabled - why store the real value ?
>
>> +
>> + /*
>> + * 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.
Right, just thought both brbcr, and brbfcr system registers represent
current BRBE state (besides branch records), in a more comprehensive
manner, although none would be changed from inside the guest.
>
>> + 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)
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