linux-kernel - Re: [PATCH 0/2] Enable GICv5 Legacy CPUIF trapping & fix TDIR cap test

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Message-ID: <864iq1otq2.wl-maz@kernel.org>
Date: Mon, 08 Dec 2025 15:53:41 +0000
From: Marc Zyngier <maz@...nel.org>
To: Sascha Bischoff <Sascha.Bischoff@....com>
Cc: "sascha.bischoff@...glemail.com" <sascha.bischoff@...glemail.com>,
	"linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>,
	"kvmarm@...ts.linux.dev"
	<kvmarm@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	nd <nd@....com>,
	"oliver.upton@...ux.dev" <oliver.upton@...ux.dev>,
	Joey Gouly
	<Joey.Gouly@....com>,
	Suzuki Poulose <Suzuki.Poulose@....com>,
	"yuzenghui@...wei.com" <yuzenghui@...wei.com>,
	"will@...nel.org"
	<will@...nel.org>
Subject: Re: [PATCH 0/2] Enable GICv5 Legacy CPUIF trapping & fix TDIR cap test

On Mon, 08 Dec 2025 15:28:22 +0000,
Sascha Bischoff <Sascha.Bischoff@....com> wrote:
> 
> These changes address two trapping-related issues when running legacy
> (i.e. GICv3) guests on GICv5 hosts.
> 
> The first change enables the vgic_v3_cpuif_trap static branch on GICv5
> hosts with legacy support, if trapping is required. The missing enable
> was caught as part of debugging why UNDEFs were being injected into
> guests when the ICH_HCR_EL2.TC bit was set - the expected bahaviour
> was that KVM should handle the trapped accesses, with the guest
> remaining blissfully unaware.
> 
> The second change fixes the specific cause of the TC bit being set in
> the first place. The test for the ICH_HCR_EL2_TDIR cap was checking
> for GICv3 CPUIF support and returning false prior to checking for
> GICv5 Legacy support. The result was that on GICv5 hosts, the test
> always returned false, and therefore the TC bit was being set. The
> issue is fixed by reordering the checks to check for GICv5 Legacy
> support first.
> 
> These changes are based against kvmarm/next.
> 
> Thanks,
> Sascha
> 
> Sascha Bischoff (2):
>   KVM: arm64: gic: Enable GICv3 CPUIF trapping on GICv5 hosts if
>     required
>   KVM: arm64: Correct test for ICH_HCR_EL2_TDIR cap for GICv5 hosts
> 
>  arch/arm64/kernel/cpufeature.c |  8 ++++----
>  arch/arm64/kvm/vgic/vgic-v3.c  | 25 +++++++++++++++----------
>  arch/arm64/kvm/vgic/vgic-v5.c  |  2 ++
>  arch/arm64/kvm/vgic/vgic.h     |  1 +
>  4 files changed, 22 insertions(+), 14 deletions(-)

Thanks for the debugging, and putting this together!

Reviewed-by: Marc Zyngier <maz@...nel.org>

	M.

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