Re: [PATCH v4 6/7] arm64: KVM: Write TRFCR value on guest switch with nVHE

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 04/01/2024 16:27, James Clark wrote:
> The guest value for TRFCR requested by the Coresight driver is saved in
> kvm_host_global_state. On guest switch this value needs to be written to
> the register. Currently TRFCR is only modified when we want to disable
> trace completely in guests due to an issue with TRBE. Expand the
> __debug_save_trace() function to always write to the register if a
> different value for guests is required, but also keep the existing TRBE
> disable behavior if that's required.
> 
> The TRFCR restore function remains functionally the same, except a value
> of 0 doesn't mean "don't restore" anymore. Now that we save both guest
> and host values the register is restored any time the guest and host
> values differ.
> 
> Signed-off-by: James Clark <james.clark@arm.com>
> ---
>   arch/arm64/kvm/hyp/nvhe/debug-sr.c | 55 ++++++++++++++++++------------
>   1 file changed, 34 insertions(+), 21 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/debug-sr.c b/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> index 4558c02eb352..7fd876d4f034 100644
> --- a/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> +++ b/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> @@ -51,32 +51,45 @@ static void __debug_restore_spe(u64 pmscr_el1)
>   	write_sysreg_s(pmscr_el1, SYS_PMSCR_EL1);
>   }
>   
> -static void __debug_save_trace(u64 *trfcr_el1)
> +/*
> + * Save TRFCR and disable trace completely if TRBE is being used, otherwise
> + * apply required guest TRFCR value.
> + */
> +static void __debug_save_trace(struct kvm_vcpu *vcpu)
>   {
> -	*trfcr_el1 = 0;
> +	u64 host_trfcr_el1 = read_sysreg_s(SYS_TRFCR_EL1);
> +	u64 guest_trfcr_el1;
> +
> +	vcpu->arch.host_debug_state.trfcr_el1 = host_trfcr_el1;
>   
>   	/* Check if the TRBE is enabled */
> -	if (!(read_sysreg_s(SYS_TRBLIMITR_EL1) & TRBLIMITR_EL1_E))
> -		return;
> -	/*
> -	 * Prohibit trace generation while we are in guest.
> -	 * Since access to TRFCR_EL1 is trapped, the guest can't
> -	 * modify the filtering set by the host.
> -	 */
> -	*trfcr_el1 = read_sysreg_s(SYS_TRFCR_EL1);
> -	write_sysreg_s(0, SYS_TRFCR_EL1);
> -	isb();
> -	/* Drain the trace buffer to memory */
> -	tsb_csync();
> +	if (vcpu_get_flag(vcpu, DEBUG_STATE_SAVE_TRBE) &&
> +	    (read_sysreg_s(SYS_TRBLIMITR_EL1) & TRBLIMITR_EL1_E)) {
> +		/*
> +		 * Prohibit trace generation while we are in guest. Since access
> +		 * to TRFCR_EL1 is trapped, the guest can't modify the filtering
> +		 * set by the host.
> +		 */
> +		write_sysreg_s(0, SYS_TRFCR_EL1);
> +		isb();
> +		/* Drain the trace buffer to memory */
> +		tsb_csync();
> +	} else {
> +		/*
> +		 * Not using TRBE, so guest trace works. Apply the guest filters
> +		 * provided by the Coresight driver, if different.
> +		 */
> +		guest_trfcr_el1 = kvm_host_global_state[vcpu->cpu].guest_trfcr_el1;
> +		if (host_trfcr_el1 != guest_trfcr_el1)
> +			write_sysreg_s(guest_trfcr_el1, SYS_TRFCR_EL1);
> +	}
>   }
>   
>   static void __debug_restore_trace(u64 trfcr_el1)
>   {
> -	if (!trfcr_el1)
> -		return;
> -
>   	/* Restore trace filter controls */
> -	write_sysreg_s(trfcr_el1, SYS_TRFCR_EL1);
> +	if (trfcr_el1 != read_sysreg_s(SYS_TRFCR_EL1))
> +		write_sysreg_s(trfcr_el1, SYS_TRFCR_EL1);

Could we not write it unconditionally here ? In the saving step, we have
to save the host setting. But while restoring, we could skip the check.
A read and write is probably the same cost, as the value is implicitly
synchronized by a later ISB.

Eitherways,

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>


>   }
>   
>   void __debug_save_host_buffers_nvhe(struct kvm_vcpu *vcpu)
> @@ -85,8 +98,8 @@ void __debug_save_host_buffers_nvhe(struct kvm_vcpu *vcpu)
>   	if (vcpu_get_flag(vcpu, DEBUG_STATE_SAVE_SPE))
>   		__debug_save_spe(&vcpu->arch.host_debug_state.pmscr_el1);
>   	/* Disable and flush Self-Hosted Trace generation */
> -	if (vcpu_get_flag(vcpu, DEBUG_STATE_SAVE_TRBE))
> -		__debug_save_trace(&vcpu->arch.host_debug_state.trfcr_el1);
> +	if (vcpu_get_flag(vcpu, DEBUG_STATE_SAVE_TRFCR))
> +		__debug_save_trace(vcpu);
>   }
>   
>   void __debug_switch_to_guest(struct kvm_vcpu *vcpu)
> @@ -98,7 +111,7 @@ void __debug_restore_host_buffers_nvhe(struct kvm_vcpu *vcpu)
>   {
>   	if (vcpu_get_flag(vcpu, DEBUG_STATE_SAVE_SPE))
>   		__debug_restore_spe(vcpu->arch.host_debug_state.pmscr_el1);
> -	if (vcpu_get_flag(vcpu, DEBUG_STATE_SAVE_TRBE))
> +	if (vcpu_get_flag(vcpu, DEBUG_STATE_SAVE_TRFCR))
>   		__debug_restore_trace(vcpu->arch.host_debug_state.trfcr_el1);
>   }
>