[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <df348434-85af-4bae-a31a-4a4a93c3cf49@arm.com>
Date: Mon, 17 Jun 2024 11:57:13 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Marc Zyngier <maz@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
will@...nel.org, catalin.marinas@....com, mark.rutland@....com,
Mark Brown <broonie@...nel.org>, James Clark <james.clark@....com>,
Rob Herring <robh@...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 2/9] KVM: arm64: Explicitly handle BRBE traps as
UNDEFINED
On 6/14/24 18:39, Marc Zyngier wrote:
> On Fri, 14 Jun 2024 13:33:37 +0100,
> Marc Zyngier <maz@...nel.org> wrote:
>>
>> On Thu, 13 Jun 2024 07:17:24 +0100,
>> Anshuman Khandual <anshuman.khandual@....com> wrote:
>>>
>>> The Branch Record Buffer Extension (BRBE) adds a number of system registers
>>> and instructions, which we don't currently intend to expose to guests. Our
>>> existing logic handles this safely, but this could be improved with some
>>> explicit handling of BRBE.
>>>
>>> The presence of BRBE is currently hidden from guests as the cpufeature
>>> code's ftr_id_aa64dfr0[] table doesn't have an entry for the BRBE field,
>>> and so this will be zero in the sanitised value of ID_AA64DFR0 exposed to
>>> guests via read_sanitised_id_aa64dfr0_el1(). As the ftr_id_aa64dfr0[] table
>>> may gain an entry for the BRBE field in future, for robustness we should
>>> explicitly mask out the BRBE field in read_sanitised_id_aa64dfr0_el1().
>>>
>>> The BRBE system registers and instructions are currently trapped by the
>>> existing configuration of the fine-grained traps. As neither the registers
>>> nor the instructions are described in the sys_reg_descs[] table,
>>> emulate_sys_reg() will warn that these are unknown before injecting an
>>> UNDEFINED exception into the guest.
>>>
>>> Well-behaved guests shouldn't try to use the registers or instructions, but
>>> badly-behaved guests could use these, resulting in unnecessary warnings. To
>>> avoid those warnings, we should explicitly handle the BRBE registers and
>>> instructions as UNDEFINED.
>>>
>>> Address the above by having read_sanitised_id_aa64dfr0_el1() mask out the
>>> ID_AA64DFR0.BRBE field, and explicitly handling all of the BRBE system
>>> registers and instructions as UNDEFINED.
>>>
>>> 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:
>>>
>>> - Updated the commit message
>>>
>>> arch/arm64/kvm/sys_regs.c | 56 +++++++++++++++++++++++++++++++++++++++
>>> 1 file changed, 56 insertions(+)
>>> Reviewed-by: Mark Rutland <mark.rutland@....com>
>>> ---
>>> arch/arm64/kvm/sys_regs.c | 56 +++++++++++++++++++++++++++++++++++++++
>>> 1 file changed, 56 insertions(+)
>>>
>>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>>> index 22b45a15d068..3d4686abe5ee 100644
>>> --- a/arch/arm64/kvm/sys_regs.c
>>> +++ b/arch/arm64/kvm/sys_regs.c
>>> @@ -1304,6 +1304,11 @@ static int set_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
>>> return 0;
>>> }
>>>
>>> +#define BRB_INF_SRC_TGT_EL1(n) \
>>> + { SYS_DESC(SYS_BRBINF_EL1(n)), undef_access }, \
>>> + { SYS_DESC(SYS_BRBSRC_EL1(n)), undef_access }, \
>>> + { SYS_DESC(SYS_BRBTGT_EL1(n)), undef_access } \
>>> +
>>> /* Silly macro to expand the DBG{BCR,BVR,WVR,WCR}n_EL1 registers in one go */
>>> #define DBG_BCR_BVR_WCR_WVR_EL1(n) \
>>> { SYS_DESC(SYS_DBGBVRn_EL1(n)), \
>>> @@ -1722,6 +1727,9 @@ static u64 read_sanitised_id_aa64dfr0_el1(struct kvm_vcpu *vcpu,
>>> /* Hide SPE from guests */
>>> val &= ~ID_AA64DFR0_EL1_PMSVer_MASK;
>>>
>>> + /* Hide BRBE from guests */
>>> + val &= ~ID_AA64DFR0_EL1_BRBE_MASK;
>>> +
>>> return val;
>>> }
>>>
>>> @@ -2240,6 +2248,52 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>>> { SYS_DESC(SYS_DBGCLAIMCLR_EL1), trap_raz_wi },
>>> { SYS_DESC(SYS_DBGAUTHSTATUS_EL1), trap_dbgauthstatus_el1 },
>>>
>>> + /*
>>> + * BRBE branch record sysreg address space is interleaved between
>>> + * corresponding BRBINF<N>_EL1, BRBSRC<N>_EL1, and BRBTGT<N>_EL1.
>>> + */
>>> + BRB_INF_SRC_TGT_EL1(0),
>>> + BRB_INF_SRC_TGT_EL1(16),
>>> + BRB_INF_SRC_TGT_EL1(1),
>>> + BRB_INF_SRC_TGT_EL1(17),
>>> + BRB_INF_SRC_TGT_EL1(2),
>>> + BRB_INF_SRC_TGT_EL1(18),
>>> + BRB_INF_SRC_TGT_EL1(3),
>>> + BRB_INF_SRC_TGT_EL1(19),
>>> + BRB_INF_SRC_TGT_EL1(4),
>>> + BRB_INF_SRC_TGT_EL1(20),
>>> + BRB_INF_SRC_TGT_EL1(5),
>>> + BRB_INF_SRC_TGT_EL1(21),
>>> + BRB_INF_SRC_TGT_EL1(6),
>>> + BRB_INF_SRC_TGT_EL1(22),
>>> + BRB_INF_SRC_TGT_EL1(7),
>>> + BRB_INF_SRC_TGT_EL1(23),
>>> + BRB_INF_SRC_TGT_EL1(8),
>>> + BRB_INF_SRC_TGT_EL1(24),
>>> + BRB_INF_SRC_TGT_EL1(9),
>>> + BRB_INF_SRC_TGT_EL1(25),
>>> + BRB_INF_SRC_TGT_EL1(10),
>>> + BRB_INF_SRC_TGT_EL1(26),
>>> + BRB_INF_SRC_TGT_EL1(11),
>>> + BRB_INF_SRC_TGT_EL1(27),
>>> + BRB_INF_SRC_TGT_EL1(12),
>>> + BRB_INF_SRC_TGT_EL1(28),
>>> + BRB_INF_SRC_TGT_EL1(13),
>>> + BRB_INF_SRC_TGT_EL1(29),
>>> + BRB_INF_SRC_TGT_EL1(14),
>>> + BRB_INF_SRC_TGT_EL1(30),
>>> + BRB_INF_SRC_TGT_EL1(15),
>>> + BRB_INF_SRC_TGT_EL1(31),
>>> +
>>> + /* Remaining BRBE sysreg addresses space */
>>> + { SYS_DESC(SYS_BRBCR_EL1), undef_access },
>>> + { SYS_DESC(SYS_BRBFCR_EL1), undef_access },
>>> + { SYS_DESC(SYS_BRBTS_EL1), undef_access },
>>> + { SYS_DESC(SYS_BRBINFINJ_EL1), undef_access },
>>> + { SYS_DESC(SYS_BRBSRCINJ_EL1), undef_access },
>>> + { SYS_DESC(SYS_BRBTGTINJ_EL1), undef_access },
>>> + { SYS_DESC(SYS_BRBIDR0_EL1), undef_access },
>>> +
>>> { SYS_DESC(SYS_MDCCSR_EL0), trap_raz_wi },
>>> { SYS_DESC(SYS_DBGDTR_EL0), trap_raz_wi },
>>> // DBGDTR[TR]X_EL0 share the same encoding
>>> @@ -2751,6 +2805,8 @@ static struct sys_reg_desc sys_insn_descs[] = {
>>> { SYS_DESC(SYS_DC_CISW), access_dcsw },
>>> { SYS_DESC(SYS_DC_CIGSW), access_dcgsw },
>>> { SYS_DESC(SYS_DC_CIGDSW), access_dcgsw },
>>> + { SYS_DESC(OP_BRB_IALL), undef_access },
>>> + { SYS_DESC(OP_BRB_INJ), undef_access },
>>> };
>>>
>>> static const struct sys_reg_desc *first_idreg;
>>
>> I don't think we need any update to the sys_reg table to handle
>> this. Instead, we should make use of the FGU infrastructure that has
>> been in since 6.9 to make this stuff UNDEF unconditionally.
>>
>> It should be as simple as:
>>
>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>> index ee33f5467ce5..7cafe3f72c01 100644
>> --- a/arch/arm64/kvm/sys_regs.c
>> +++ b/arch/arm64/kvm/sys_regs.c
>> @@ -4964,6 +4964,11 @@ void kvm_init_sysreg(struct kvm_vcpu *vcpu)
>> kvm->arch.fgu[HAFGRTR_GROUP] |= ~(HAFGRTR_EL2_RES0 |
>> HAFGRTR_EL2_RES1);
>>
>> + if (!kvm_has_feat(kvm, ID_AA64DFR0_EL1, BRBE, IMP))
>> + kvm->arch.fgu[HDFGRTR_GROUP] |= (HDFGRTR_nBRBDATA |
>> + HDFGRTR_nBRBCTL |
>> + HDFGRTR_nBRBIDR);
>> +
>> set_bit(KVM_ARCH_FLAG_FGU_INITIALIZED, &kvm->arch.flags);
>> out:
>> mutex_unlock(&kvm->arch.config_lock);
>>
>> which is of course untested, but that I expect to be correct.
>
> Actually, to disable the *instructions*, a similar hack must be
> applied to HFGITR_EL2. The resulting patch should be something like:
>
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index ee33f5467ce5..49d86dae8d80 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -4964,6 +4964,15 @@ void kvm_init_sysreg(struct kvm_vcpu *vcpu)
> kvm->arch.fgu[HAFGRTR_GROUP] |= ~(HAFGRTR_EL2_RES0 |
> HAFGRTR_EL2_RES1);
>
> + if (!kvm_has_feat(kvm, ID_AA64DFR0_EL1, BRBE, IMP)) {
> + kvm->arch.fgu[HDFGRTR_GROUP] |= (HDFGRTR_nBRBDATA |
> + HDFGRTR_nBRBCTL |
> + HDFGRTR_nBRBIDR);
> + kvm->arch.fgu[HFGITR_GROUP] |= (HFGITR_EL2_nBRBINJ |
> + HFGITR_EL2_nBRBIALL);
> + }
> +
> +
> set_bit(KVM_ARCH_FLAG_FGU_INITIALIZED, &kvm->arch.flags);
> out:
> mutex_unlock(&kvm->arch.config_lock);
This makes sense, will remove all the changes to sys_reg table and
instead fold the above suggestion into the patch.
>
> The implicit dependency here is that FGT is always present on a system
> that implements BRBE. The architecture supports this assertion:
>
> - BRBE is not available before ARMv9.1
> - FGT is mandatory from ARMv8.6
>
> Given that v9.1 is congruent to v8.6, we have the required overlap.
So this overlap need not be asserted in software again ?
Powered by blists - more mailing lists