Re: [PATCH V16 5/8] KVM: arm64: nvhe: Disable branch generation in nVHE guests

[Marc Zyngier <maz@kernel.org>]

On Thu, 25 Jan 2024 09:41:16 +0000,
Anshuman Khandual <anshuman.khandual@arm.com> wrote:
> 
> Disable the BRBE before we enter the guest, saving the status and enable it
> back once we get out of the guest. This avoids capturing branch records in
> the guest kernel or userspace, which would be confusing the host samples.
> 
> 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 V16:
> 
> - Dropped BRBCR_EL1 and BRBFCR_EL1 from enum vcpu_sysreg
> - Reverted back the KVM NVHE patch - used host_debug_state based 'brbcr_el1'
>   element, and dropped the previous dependency on Jame's coresight series
> 
>  arch/arm64/include/asm/kvm_host.h  |  5 ++++-
>  arch/arm64/kvm/debug.c             |  5 +++++
>  arch/arm64/kvm/hyp/nvhe/debug-sr.c | 33 ++++++++++++++++++++++++++++++
>  3 files changed, 42 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 21c57b812569..bce8792092af 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -569,7 +569,7 @@ struct kvm_vcpu_arch {
>  	u8 cflags;
>  
>  	/* Input flags to the hypervisor code, potentially cleared after use */
> -	u8 iflags;
> +	u16 iflags;
>  
>  	/* State flags for kernel bookkeeping, unused by the hypervisor code */
>  	u8 sflags;
> @@ -610,6 +610,7 @@ struct kvm_vcpu_arch {
>  		u64 pmscr_el1;
>  		/* Self-hosted trace */
>  		u64 trfcr_el1;
> +		u64 brbcr_el1;
>  	} host_debug_state;
>  
>  	/* VGIC state */
> @@ -779,6 +780,8 @@ struct kvm_vcpu_arch {
>  #define DEBUG_STATE_SAVE_TRBE	__vcpu_single_flag(iflags, BIT(6))
>  /* vcpu running in HYP context */
>  #define VCPU_HYP_CONTEXT	__vcpu_single_flag(iflags, BIT(7))
> +/* Save BRBE context if active  */
> +#define DEBUG_STATE_SAVE_BRBE	__vcpu_single_flag(iflags, BIT(8))
>  
>  /* SVE enabled for host EL0 */
>  #define HOST_SVE_ENABLED	__vcpu_single_flag(sflags, BIT(0))
> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> index 8725291cb00a..99f85d8acbf3 100644
> --- a/arch/arm64/kvm/debug.c
> +++ b/arch/arm64/kvm/debug.c
> @@ -335,10 +335,15 @@ void kvm_arch_vcpu_load_debug_state_flags(struct kvm_vcpu *vcpu)
>  	if (cpuid_feature_extract_unsigned_field(dfr0, ID_AA64DFR0_EL1_TraceBuffer_SHIFT) &&
>  	    !(read_sysreg_s(SYS_TRBIDR_EL1) & TRBIDR_EL1_P))
>  		vcpu_set_flag(vcpu, DEBUG_STATE_SAVE_TRBE);
> +
> +	/* Check if we have BRBE implemented and available at the host */
> +	if (cpuid_feature_extract_unsigned_field(dfr0, ID_AA64DFR0_EL1_BRBE_SHIFT))
> +		vcpu_set_flag(vcpu, DEBUG_STATE_SAVE_BRBE);
>  }
>  
>  void kvm_arch_vcpu_put_debug_state_flags(struct kvm_vcpu *vcpu)
>  {
>  	vcpu_clear_flag(vcpu, DEBUG_STATE_SAVE_SPE);
>  	vcpu_clear_flag(vcpu, DEBUG_STATE_SAVE_TRBE);
> +	vcpu_clear_flag(vcpu, DEBUG_STATE_SAVE_BRBE);
>  }
> diff --git a/arch/arm64/kvm/hyp/nvhe/debug-sr.c b/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> index 4558c02eb352..79bcf0fb1326 100644
> --- a/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> +++ b/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> @@ -79,6 +79,34 @@ static void __debug_restore_trace(u64 trfcr_el1)
>  	write_sysreg_s(trfcr_el1, SYS_TRFCR_EL1);
>  }
>  
> +static void __debug_save_brbe(u64 *brbcr_el1)
> +{
> +	*brbcr_el1 = 0;
> +
> +	/* Check if the BRBE is enabled */
> +	if (!(read_sysreg_s(SYS_BRBCR_EL1) & (BRBCR_ELx_E0BRE | BRBCR_ELx_ExBRE)))
> +		return;
> +
> +	/*
> +	 * Prohibit branch record generation while we are in guest.
> +	 * Since access to BRBCR_EL1 is trapped, the guest can't
> +	 * modify the filtering set by the host.
> +	 */
> +	*brbcr_el1 = read_sysreg_s(SYS_BRBCR_EL1);
> +	write_sysreg_s(0, SYS_BRBCR_EL1);

As for TRFCR and PMSCR, this is broken on hVHE.

Please see [1]

	M.

[1] https://lore.kernel.org/r/20240229145417.3606279-1-maz@kernel.org

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