Re: [PATCH V18 2/9] KVM: arm64: Explicitly handle BRBE traps as UNDEFINED

[Marc Zyngier <maz@kernel.org>]

On Thu, 13 Jun 2024 07:17:24 +0100,
Anshuman Khandual <anshuman.khandual@arm.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@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 V18:
> 
> - Updated the commit message
> 
>  arch/arm64/kvm/sys_regs.c | 56 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 56 insertions(+)
> Reviewed-by: Mark Rutland <mark.rutland@arm.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.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.