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Mon, 24 Feb 2025 08:32:19 +0000 Date: Mon, 24 Feb 2025 08:32:18 +0000 Message-ID: <86msebrbe5.wl-maz@kernel.org> From: Marc Zyngier To: Aneesh Kumar K.V Cc: kvmarm@lists.linux.dev, kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Joey Gouly , Suzuki K Poulose , Oliver Upton , Zenghui Yu , Eric Auger Subject: Re: [PATCH 03/14] KVM: arm64: Mark HCR.EL2.E2H RES0 when ID_AA64MMFR1_EL1.VH is zero In-Reply-To: References: <20250215173816.3767330-1-maz@kernel.org> <20250215173816.3767330-4-maz@kernel.org> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/29.4 (aarch64-unknown-linux-gnu) MULE/6.0 (HANACHIRUSATO) MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=US-ASCII X-SA-Exim-Connect-IP: 185.219.108.64 X-SA-Exim-Rcpt-To: aneesh.kumar@kernel.org, kvmarm@lists.linux.dev, kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, joey.gouly@arm.com, suzuki.poulose@arm.com, oliver.upton@linux.dev, yuzenghui@huawei.com, eric.auger@redhat.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250224_003222_453258_38ED20DF X-CRM114-Status: GOOD ( 19.09 ) 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 Mon, 24 Feb 2025 07:39:30 +0000, Aneesh Kumar K.V wrote: > > Marc Zyngier writes: > > > Enforce HCR_EL2.E2H being RES0 when VHE is disabled, so that we can > > actually rely on that bit never being flipped behind our back. > > > > Signed-off-by: Marc Zyngier > > --- > > arch/arm64/kvm/nested.c | 2 ++ > > 1 file changed, 2 insertions(+) > > > > diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c > > index 0c9387d2f5070..ed3add7d32f66 100644 > > --- a/arch/arm64/kvm/nested.c > > +++ b/arch/arm64/kvm/nested.c > > @@ -1034,6 +1034,8 @@ int kvm_init_nv_sysregs(struct kvm_vcpu *vcpu) > > res0 |= (HCR_TEA | HCR_TERR); > > if (!kvm_has_feat(kvm, ID_AA64MMFR1_EL1, LO, IMP)) > > res0 |= HCR_TLOR; > > + if (!kvm_has_feat(kvm, ID_AA64MMFR1_EL1, VH, IMP)) > > + res0 |= HCR_E2H; > > if (!kvm_has_feat(kvm, ID_AA64MMFR4_EL1, E2H0, IMP)) > > res1 |= HCR_E2H; > > > > Does it make sense to check for E2H0 if MMFR1_EL1.VH == 0 ? > Should the above check be > if (!kvm_has_feat(kvm, ID_AA64MMFR1_EL1, VH, IMP)) > res0 |= HCR_E2H; > else if (!kvm_has_feat(kvm, ID_AA64MMFR4_EL1, E2H0, IMP)) > res1 |= HCR_E2H; What difference does it make? This bit can only have a reserved value, and can never be actively modified. If you *really* wanted to optimise this for reasons that I really cannot fathom, you could have this: if (!kvm_has_feat(kvm, ID_AA64MMFR1_EL1, VH, IMP)) res0 |= HCR_E2H; else res1 |= HCR_E2H; because that's what we really implement. Does it matter? I don't think so. If anything, I'd rather we keep the code as is and have a run-time warning if a bit is simultaneously RES0 and RES1, because that'd be indicative of a much bigger problem. Thanks, M. -- Without deviation from the norm, progress is not possible.