linux-kernel - Re: [PATCH V15 2/8] KVM: arm64: Prevent guest accesses into BRBE system registers/instructions

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Message-ID: <86v89ebcn2.wl-maz@kernel.org>
Date:   Mon, 04 Dec 2023 08:22:25 +0000
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 V15 2/8] KVM: arm64: Prevent guest accesses into BRBE system registers/instructions

On Fri, 01 Dec 2023 05:39:00 +0000,
Anshuman Khandual <anshuman.khandual@....com> wrote:
> 
> Currently BRBE feature is not supported in a guest environment. This hides
> BRBE feature availability via masking ID_AA64DFR0_EL1.BRBE field. This also
> blocks guest accesses into BRBE system registers and instructions as if the
> underlying hardware never implemented FEAT_BRBE feature.
> 
> 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>
> ---
>  arch/arm64/kvm/sys_regs.c | 130 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 130 insertions(+)
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 4735e1b37fb3..42701065b3cd 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1583,6 +1583,9 @@ 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;
>  }
>  
> @@ -2042,6 +2045,8 @@ static const struct sys_reg_desc sys_reg_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 },
>  
>  	DBG_BCR_BVR_WCR_WVR_EL1(0),
>  	DBG_BCR_BVR_WCR_WVR_EL1(1),
> @@ -2072,6 +2077,131 @@ 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.
> +	 */
> +	{ SYS_DESC(SYS_BRBINF0_EL1), undef_access },
> +	{ SYS_DESC(SYS_BRBSRC0_EL1), undef_access },
> +	{ SYS_DESC(SYS_BRBTGT0_EL1), undef_access },
> +	{ SYS_DESC(SYS_BRBINF16_EL1), undef_access },
> +	{ SYS_DESC(SYS_BRBSRC16_EL1), undef_access },
> +	{ SYS_DESC(SYS_BRBTGT16_EL1), undef_access },

Surely we can do better than this wall of text. Please look at what we
do for the debug registers, and adopt a similar pattern. This should
result in one line per group of 3 registers.

What is the plan for KVM support beyond this?

	M.

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