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Date: Mon, 26 Feb 2024 18:26:53 +0000
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From: Marc Zyngier <maz@kernel.org>
To: James Clark <james.clark@arm.com>
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Subject: Re: [PATCH v6 7/8] arm64: KVM: Write TRFCR value on guest switch with nVHE
In-Reply-To: <20240226113044.228403-8-james.clark@arm.com>
References: <20240226113044.228403-1-james.clark@arm.com>
	<20240226113044.228403-8-james.clark@arm.com>
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On Mon, 26 Feb 2024 11:30:35 +0000,
James Clark <james.clark@arm.com> wrote:
> 
> The guest value for TRFCR requested by the Coresight driver is saved in
> kvm_guest_trfcr. 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.
> 
> In pKVM, the kvm_guest_trfcr can't be read and the host isn't trusted,
> so always disable trace.
> 
> __debug_restore_trace() now has to restore unconditionally, because even
> a value of 0 needs to be written to overwrite whatever was set for the
> guest.
> 
> Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> Signed-off-by: James Clark <james.clark@arm.com>
> ---
>  arch/arm64/kvm/hyp/nvhe/debug-sr.c | 53 +++++++++++++++++-------------
>  1 file changed, 31 insertions(+), 22 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/debug-sr.c b/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> index 4558c02eb352..3adac2e01908 100644
> --- a/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> +++ b/arch/arm64/kvm/hyp/nvhe/debug-sr.c
> @@ -51,30 +51,39 @@ static void __debug_restore_spe(u64 pmscr_el1)
>  	write_sysreg_s(pmscr_el1, SYS_PMSCR_EL1);
>  }
>  
> -static void __debug_save_trace(u64 *trfcr_el1)
> +static void __debug_save_trace(struct kvm_vcpu *vcpu)
>  {
> -	*trfcr_el1 = 0;
> -
> -	/* 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();
> +	u64 host_trfcr_el1 = read_sysreg_s(SYS_TRFCR_EL1);

Ah, and on top of that, this is already broken with hVHE.

I'll send a patch.

	M.

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