Messages in this thread | | | Date | Mon, 27 Nov 2023 13:55:22 +0530 | Subject | Re: [V14 5/8] KVM: arm64: nvhe: Disable branch generation in nVHE guests | From | Anshuman Khandual <> |
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On 11/23/23 19:24, James Clark wrote: > > > On 21/11/2023 11:12, Anshuman Khandual wrote: >> >> >> 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@kernel.org> >>>> Cc: Oliver Upton <oliver.upton@linux.dev> >>>> Cc: James Morse <james.morse@arm.com> >>>> Cc: Suzuki K Poulose <suzuki.poulose@arm.com> >>>> Cc: Catalin Marinas <catalin.marinas@arm.com> >>>> Cc: Will Deacon <will@kernel.org> >>>> Cc: kvmarm@lists.linux.dev >>>> Cc: linux-arm-kernel@lists.infradead.org >>>> CC: linux-kernel@vger.kernel.org >>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.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 ? >> > > It's fewer lines of code and it's less likely to catch someone out if > it's always set to whatever the host value was. Using 0 as a special > value could also be an issue because it's indistinguishable from if the > register was actually set to 0. It's just more to reason about when you > could reduce it to a single assignment. > > Also it probably would have avoided the current mistake if it was always > assigned to the host value as well.
Okay, will always save SYS_BRBCR_EL1 into ctxt_sys_reg(host_ctxt, BRBCR_EL1).
> >>> >>>> + >>>> + /* >>>> + * 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. >> > > The comment above doesn't match up with this explanation. > > Having it in the code implies that it's needed. And as you say the > branch records are missing anyway, so you can't even infer that it's > only done to be comprehensive. > > It would be better to not make anyone reading it wonder why it's done > and just not do it. It's only 8 bytes but it's also a waste of space.
Sure, will drop BRBFCR_EL1 handling in here. The changed code is something like as follows.
static void __debug_save_brbe(struct kvm_cpu_context *host_ctxt) { ctxt_sys_reg(host_ctxt, BRBCR_EL1) = read_sysreg_s(SYS_BRBCR_EL1);
/* Check if the BRBE is enabled */ if (!(ctxt_sys_reg(host_ctxt, BRBCR_EL1) & (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. */ write_sysreg_s(0, SYS_BRBCR_EL1); isb(); }
static void __debug_restore_brbe(struct kvm_cpu_context *host_ctxt) { if (!ctxt_sys_reg(host_ctxt, BRBCR_EL1)) return;
/* Restore BRBE controls */ write_sysreg_s(ctxt_sys_reg(host_ctxt, BRBCR_EL1), SYS_BRBCR_EL1); isb(); }
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