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Sat, 25 Sep 2021 09:50:08 +0000 (UTC) Received: from sofa.misterjones.org ([185.219.108.64] helo=wait-a-minute.misterjones.org) by disco-boy.misterjones.org with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.94.2) (envelope-from ) id 1mU4Jq-00Cuoe-EB; Sat, 25 Sep 2021 10:50:06 +0100 Date: Sat, 25 Sep 2021 10:50:05 +0100 Message-ID: <878rzlass2.wl-maz@kernel.org> From: Marc Zyngier To: Sean Christopherson Cc: Huacai Chen , Aleksandar Markovic , Paul Mackerras , Christian Borntraeger , Janosch Frank , Paolo Bonzini , James Morse , Alexandru Elisei , Suzuki K Poulose , David Hildenbrand , Cornelia Huck , Claudio Imbrenda , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-mips@vger.kernel.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, linux-kernel@vger.kernel.org, David Matlack , Jing Zhang Subject: Re: [PATCH 07/14] KVM: Don't block+unblock when halt-polling is successful In-Reply-To: <20210925005528.1145584-8-seanjc@google.com> References: <20210925005528.1145584-1-seanjc@google.com> <20210925005528.1145584-8-seanjc@google.com> 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/27.1 (x86_64-pc-linux-gnu) MULE/6.0 (HANACHIRUSATO) MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") X-SA-Exim-Connect-IP: 185.219.108.64 X-SA-Exim-Rcpt-To: seanjc@google.com, chenhuacai@kernel.org, aleksandar.qemu.devel@gmail.com, paulus@ozlabs.org, borntraeger@de.ibm.com, frankja@linux.ibm.com, pbonzini@redhat.com, james.morse@arm.com, alexandru.elisei@arm.com, suzuki.poulose@arm.com, david@redhat.com, cohuck@redhat.com, imbrenda@linux.ibm.com, vkuznets@redhat.com, wanpengli@tencent.com, jmattson@google.com, joro@8bytes.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-mips@vger.kernel.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, linux-kernel@vger.kernel.org, dmatlack@google.com, jingzhangos@google.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-20210925_025009_483482_4FADDA09 X-CRM114-Status: GOOD ( 22.88 ) 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: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Sat, 25 Sep 2021 01:55:21 +0100, Sean Christopherson wrote: > > Invoke the arch hooks for block+unblock if and only if KVM actually > attempts to block the vCPU. The only non-nop implementation is on arm64, > and if halt-polling is successful, there is no need for arm64 to put/load > the vGIC as KVM hasn't relinquished control of the vCPU in any way. This doesn't mean that there is no requirement for any state change. The put/load on GICv4 is crucial for performance, and the VMCR resync is a correctness requirement. > > The primary motivation is to allow future cleanup to split out "block" > from "halt", but this is also likely a small performance boost on arm64 > when halt-polling is successful. > > Adjust the post-block path to update "cur" after unblocking, i.e. include > vGIC load time in halt_wait_ns and halt_wait_hist, so that the behavior > is consistent. Moving just the pre-block arch hook would result in only > the vGIC put latency being included in the halt_wait stats. There is no > obvious evidence that one way or the other is correct, so just ensure KVM > is consistent. This effectively reverts 07ab0f8d9a12 ("KVM: Call kvm_arch_vcpu_blocking early into the blocking sequence"), which was a huge gain on arm64, not to mention a correctness fix. Without this, a GICv4 machine will always pay for the full poll penalty, going into schedule(), and only then get a doorbell interrupt signalling telling the kernel that there was an interrupt. On a non-GICv4 machine, it means that interrupts injected by another thread during the pooling will be evaluated with an outdated priority mask, which can result in either a spurious wake-up or a missed wake-up. If it means introducing a new set of {pre,post}-poll arch-specific hooks, so be it. But I don't think this change is acceptable as is. Thanks, M. -- Without deviation from the norm, progress is not possible. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel