linux-kernel - Re: [PATCH v2 10/43] KVM: arm64: Move vGIC v4 handling for WFI out arch callback hook

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Date:   Tue, 26 Oct 2021 16:41:05 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Anup Patel <anup.patel@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Atish Patra <atish.patra@....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, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        David Matlack <dmatlack@...gle.com>,
        Oliver Upton <oupton@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>
Subject: Re: [PATCH v2 10/43] KVM: arm64: Move vGIC v4 handling for WFI out arch callback hook

On Mon, 25 Oct 2021 14:31:48 +0100,
Paolo Bonzini <pbonzini@...hat.com> wrote:
> 
> On 09/10/21 04:12, Sean Christopherson wrote:
> > Move the put and reload of the vGIC out of the block/unblock callbacks
> > and into a dedicated WFI helper.  Functionally, this is nearly a nop as
> > the block hook is called at the very beginning of kvm_vcpu_block(), and
> > the only code in kvm_vcpu_block() after the unblock hook is to update the
> > halt-polling controls, i.e. can only affect the next WFI.
> > 
> > Back when the arch (un)blocking hooks were added by commits 3217f7c25bca
> > ("KVM: Add kvm_arch_vcpu_{un}blocking callbacks) and d35268da6687
> > ("arm/arm64: KVM: arch_timer: Only schedule soft timer on vcpu_block"),
> > the hooks were invoked only when KVM was about to "block", i.e. schedule
> > out the vCPU.  The use case at the time was to schedule a timer in the
> > host based on the earliest timer in the guest in order to wake the
> > blocking vCPU when the emulated guest timer fired.  Commit accb99bcd0ca
> > ("KVM: arm/arm64: Simplify bg_timer programming") reworked the timer
> > logic to be even more precise, by waiting until the vCPU was actually
> > scheduled out, and so move the timer logic from the (un)blocking hooks to
> > vcpu_load/put.
> > 
> > In the meantime, the hooks gained usage for enabling vGIC v4 doorbells in
> > commit df9ba95993b9 ("KVM: arm/arm64: GICv4: Use the doorbell interrupt
> > as an unblocking source"), and added related logic for the VMCR in commit
> > 5eeaf10eec39 ("KVM: arm/arm64: Sync ICH_VMCR_EL2 back when about to block").
> > 
> > Finally, commit 07ab0f8d9a12 ("KVM: Call kvm_arch_vcpu_blocking early
> > into the blocking sequence") hoisted the (un)blocking hooks so that they
> > wrapped KVM's halt-polling logic in addition to the core "block" logic.
> > 
> > In other words, the original need for arch hooks to take action _only_
> > in the block path is long since gone.
> > 
> > Cc: Oliver Upton <oupton@...gle.com>
> > Cc: Marc Zyngier <maz@...nel.org>
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> 
> This needs a word on why kvm_psci_vcpu_suspend does not need the
> hooks.  Or it needs to be changed to also use kvm_vcpu_wfi in the PSCI
> code, I don't know.
> 
> Marc, can you review and/or advise?

I was looking at that over the weekend, and that's a pre-existing
bug. I would have addressed it independently, but it looks like you
already have queued the patch.

I guess I'll have to revisit this once the whole thing lands
somewhere.

	M.

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