linux-kernel - Re: [PATCH v19 10/11] KVM: arm64: nvhe: Disable branch generation in nVHE guests

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Message-ID: <20250213170316.GG235556@e132581.arm.com>
Date: Thu, 13 Feb 2025 17:03:16 +0000
From: Leo Yan <leo.yan@....com>
To: "Rob Herring (Arm)" <robh@...nel.org>
Cc: Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>,
	Joey Gouly <joey.gouly@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	James Clark <james.clark@...aro.org>,
	Anshuman Khandual <anshuman.khandual@....com>,
	linux-arm-kernel@...ts.infradead.org,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, kvmarm@...ts.linux.dev
Subject: Re: [PATCH v19 10/11] KVM: arm64: nvhe: Disable branch generation in
 nVHE guests

On Sun, Feb 02, 2025 at 06:43:04PM -0600, Rob Herring (Arm) wrote:

[...]

> +static void __debug_save_brbe(u64 *brbcr_el1)
> +{
> +       *brbcr_el1 = 0;
> +
> +       /* Check if the BRBE is enabled */
> +       if (!(read_sysreg_el1(SYS_BRBCR) & (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_el1(SYS_BRBCR);
> +       write_sysreg_el1(0, SYS_BRBCR);
> +}

Should flush branch record and use isb() before exit host kernel?

I see inconsistence between the function above and BRBE's disable
function. Here it clears E0BRE / ExBRE bits for disabling BRBE, but the
BRBE driver sets the PAUSED bit in BRBFCR_EL1 for disabling BRBE.

Thanks,
Leo