Re: [PATCH v20 10/11] KVM: arm64: nvhe: Disable branch generation in nVHE guests

Leo Yan
2025-02-24T10:41:50Z

On Tue, Feb 18, 2025 at 02:40:05PM -0600, Rob Herring (Arm) wrote:
>
> From: Anshuman Khandual <anshuman.khandual@arm.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@arm.com>
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Co-developed-by: Rob Herring (Arm) <robh@kernel.org>
> Signed-off-by: Rob Herring (Arm) <robh@kernel.org>

Reviewed-by: Leo Yan <leo.yan@arm.com>