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Date: Fri, 14 Jun 2024 14:09:38 +0100
From: Marc Zyngier <maz@...nel.org>
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.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 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);
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.
Thanks,
M.
--
Without deviation from the norm, progress is not possible.
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