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Message-ID: <878rzlass2.wl-maz@kernel.org> Date: Sat, 25 Sep 2021 10:50:05 +0100 From: Marc Zyngier <maz@...nel.org> To: Sean Christopherson <seanjc@...gle.com> Cc: Huacai Chen <chenhuacai@...nel.org>, Aleksandar Markovic <aleksandar.qemu.devel@...il.com>, Paul Mackerras <paulus@...abs.org>, Christian Borntraeger <borntraeger@...ibm.com>, Janosch Frank <frankja@...ux.ibm.com>, Paolo Bonzini <pbonzini@...hat.com>, James Morse <james.morse@....com>, Alexandru Elisei <alexandru.elisei@....com>, Suzuki K Poulose <suzuki.poulose@....com>, David Hildenbrand <david@...hat.com>, Cornelia Huck <cohuck@...hat.com>, Claudio Imbrenda <imbrenda@...ux.ibm.com>, Vitaly Kuznetsov <vkuznets@...hat.com>, Wanpeng Li <wanpengli@...cent.com>, Jim Mattson <jmattson@...gle.com>, Joerg Roedel <joro@...tes.org>, linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu, linux-mips@...r.kernel.org, kvm@...r.kernel.org, kvm-ppc@...r.kernel.org, linux-kernel@...r.kernel.org, David Matlack <dmatlack@...gle.com>, Jing Zhang <jingzhangos@...gle.com> Subject: Re: [PATCH 07/14] KVM: Don't block+unblock when halt-polling is successful On Sat, 25 Sep 2021 01:55:21 +0100, Sean Christopherson <seanjc@...gle.com> 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.
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