linux-kernel - Re: [PATCH v22 4/5] KVM: arm64: nvhe: Disable branch generation in nVHE guests

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Message-ID: <c057dcd5-6a75-4689-8812-6e62bf79e9b1@arm.com>
Date: Wed, 21 May 2025 10:36:56 +0100
From: Suzuki K Poulose <suzuki.poulose@....com>
To: "Rob Herring (Arm)" <robh@...nel.org>, 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>, Zenghui Yu <yuzenghui@...wei.com>,
 James Clark <james.clark@...aro.org>,
 Anshuman Khandual <anshuman.khandual@....com>, Leo Yan <leo.yan@....com>
Cc: 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 v22 4/5] KVM: arm64: nvhe: Disable branch generation in
 nVHE guests

On 20/05/2025 23:27, Rob Herring (Arm) wrote:
> From: Anshuman Khandual <anshuman.khandual@....com>
> 
> While BRBE can record branches within guests, the host recording
> branches in guests is not supported by perf (though events are).
> Support for BRBE in guests will supported by providing direct access
> to BRBE within the guests. That is how x86 LBR works for guests.
> Therefore, BRBE needs to be disabled on guest entry and restored on
> exit.
> 
> For nVHE, this requires explicit handling for guests. Before
> entering a guest, save the BRBE state and disable the it. When
> returning to the host, restore the state.
> 
> For VHE, it is not necessary. We initialize
> BRBCR_EL1.{E1BRE,E0BRE}=={0,0} at boot time, and HCR_EL2.TGE==1 while
> running in the host. We configure BRBCR_EL2.{E2BRE,E0HBRE} to enable
> branch recording in the host. When entering the guest, we set
> HCR_EL2.TGE==0 which means BRBCR_EL1 is used instead of BRBCR_EL2.
> Consequently for VHE, BRBE recording is disabled at EL1 and EL0 when
> running a guest.
> 
> Should recording in guests (by the host) ever be desired, the perf ABI
> will need to be extended to distinguish guest addresses (struct
> perf_branch_entry.priv) for starters. BRBE records would also need to be
> invalidated on guest entry/exit as guest/host EL1 and EL0 records can't
> be distinguished.
> 
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> Co-developed-by: Rob Herring (Arm) <robh@...nel.org>
> Tested-by: James Clark <james.clark@...aro.org>
> Reviewed-by: Leo Yan <leo.yan@....com>
> Signed-off-by: Rob Herring (Arm) <robh@...nel.org>

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