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From: Oliver Upton To: Marc Zyngier Cc: kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org, James Morse , Suzuki K Poulose , Zenghui Yu , Alexander Potapenko Subject: Re: [PATCH 06/12] KVM: arm64: Zero ID_AA64PFR0_EL1.GIC when no GICv3 is presented to the guest Message-ID: References: <20240820100349.3544850-1-maz@kernel.org> <20240820100349.3544850-7-maz@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240820100349.3544850-7-maz@kernel.org> X-Migadu-Flow: FLOW_OUT X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240820_165028_162628_E8752487 X-CRM114-Status: GOOD ( 20.61 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Aug 20, 2024 at 11:03:43AM +0100, Marc Zyngier wrote: > In order to be consistent, we shouldn't advertise a GICv3 when none > is actually usable by the guest. > > Wipe the feature when these conditions apply, and allow the field > to be written from userspace. > > Signed-off-by: Marc Zyngier > --- > arch/arm64/kvm/sys_regs.c | 6 +++++- > 1 file changed, 5 insertions(+), 1 deletion(-) > > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c > index bc2d54da3827..7d00d7e359e1 100644 > --- a/arch/arm64/kvm/sys_regs.c > +++ b/arch/arm64/kvm/sys_regs.c > @@ -2365,7 +2365,6 @@ static const struct sys_reg_desc sys_reg_descs[] = { > ID_AA64PFR0_EL1_MPAM | > ID_AA64PFR0_EL1_SVE | > ID_AA64PFR0_EL1_RAS | > - ID_AA64PFR0_EL1_GIC | > ID_AA64PFR0_EL1_AdvSIMD | > ID_AA64PFR0_EL1_FP), }, > ID_SANITISED(ID_AA64PFR1_EL1), > @@ -4634,6 +4633,11 @@ int kvm_finalize_sys_regs(struct kvm_vcpu *vcpu) > > guard(mutex)(&kvm->arch.config_lock); > > + if (!kvm_has_gicv3(kvm)) { > + kvm->arch.id_regs[IDREG_IDX(SYS_ID_AA64PFR0_EL1)] &= ~ID_AA64PFR0_EL1_GIC_MASK; > + kvm->arch.id_regs[IDREG_IDX(SYS_ID_PFR1_EL1)] &= ~ID_PFR1_EL1_GIC_MASK; > + } > + Hmm, should we use the ID register field as the source of truth for kvm_has_gicv3() at this point? I think what you have in patch #1 makes good sense for a stable backport. Using the ID register from this point forward would make the behavior consistent for a stupid userspace instantiated GICv3 for the VM but clobbered it from the ID register. AFAICT all other usage of kvm_has_gicv3() happens after kvm_finalize_sys_regs(), so it should take this last-minute fixup into account. -- Thanks, Oliver