Re: [PATCH V17 2/9] KVM: arm64: Explicitly handle BRBE traps as UNDEFINED

[Mark Rutland]

Hi Anshuman,

On Fri, Apr 05, 2024 at 08:16:32AM +0530, Anshuman Khandual 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
> are not described in the sys_reg_descs[] nor the instructions are described
> in the sys_insn_descs[] table, emulate_sys_reg() will warn that these are
> unknown before injecting an UNDEFINED exception into the guest.

That last sentence doesn't make sense, and I think it has been mangled.
My suggested text in v16 was:

| As the registers and instructions are not described in the
| sys_reg_descs[] table, emulate_sys_reg() will warn that these are
| unknown before injecting an UNDEFINED exception into the guest.

.. and I'd be happy with changing that to:

| 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.

I see that I had mangled this sentence originally -- thanks for
correcting that; this looks fine to me.

> To avoid those warnings, we should explicitly handle the BRBE
> registers, and instructions as UNDEFINED.

I think the added comma is unnecessary and makes this hard to read. My
suggested text in v16 was:

| 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 by adding sys_reg_descs entries for all of the
> BRBE system registers, also by adding sys_insn_descs entries for all of the
> BRBE instructions, treating these all as UNDEFINED.

Similarly, I think this was clearer as I originally suggested:

| Address the above by having read_sanitised_id_aa64dfr0_el1() mask out
| the ID_AA64DFR0.BRBE field, and by adding sys_reg_desc entries for all
| of the BRBE system registers and instructions, treating these all as
| UNDEFINED.

.. or we can simplify that to:

| 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@xxxxxxxxxx>
> Cc: Oliver Upton <oliver.upton@xxxxxxxxx>
> Cc: James Morse <james.morse@xxxxxxx>
> Cc: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: kvmarm@xxxxxxxxxxxxxxx
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> ----
> Changes in V17:
> 
> - Updated the commit message including about sys_insn_descs[]
> - Changed KVM to use existing SYS_BRBSRC/TGT/INF_EL1(n) format
> - Moved the BRBE instructions into sys_insn_descs[] array

Aside from my nits on the commit message above, these changes all look
good to me. So with the commit message cleaned up as above:

Reviewed-by: Mark Rutland <mark.rutland@xxxxxxx>

Mark.

> 
>  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 c9f4f387155f..3981aa32c5a3 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)),					\
> @@ -1708,6 +1713,9 @@ 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;
>  }
>  
> @@ -2226,6 +2234,52 @@ 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
> @@ -2738,6 +2792,8 @@ 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;
> -- 
> 2.25.1
>