linux-kernel - Re: [PATCH v2 00/43] KVM: Halt-polling and x86 APICv overhaul

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Date:   Tue, 26 Oct 2021 20:29:57 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>, 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>,
        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>,
        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 00/43] KVM: Halt-polling and x86 APICv overhaul



Am 26.10.21 um 16:48 schrieb Sean Christopherson:
> On Tue, Oct 26, 2021, Christian Borntraeger wrote:
>> Am 09.10.21 um 04:11 schrieb Sean Christopherson:
>>> This is basically two series smushed into one.  The first "half" aims
>>> to differentiate between "halt" and a more generic "block", where "halt"
>>> aligns with x86's HLT instruction, the halt-polling mechanisms, and
>>> associated stats, and "block" means any guest action that causes the vCPU
>>> to block/wait.
>>>
>>> The second "half" overhauls x86's APIC virtualization code (Posted
>>> Interrupts on Intel VMX, AVIC on AMD SVM) to do their updates in response
>>> to vCPU (un)blocking in the vcpu_load/put() paths, keying off of the
>>> vCPU's rcuwait status to determine when a blocking vCPU is being put and
>>> reloaded.  This idea comes from arm64's kvm_timer_vcpu_put(), which I
>>> stumbled across when diving into the history of arm64's (un)blocking hooks.
>>>
>>> The x86 APICv overhaul allows for killing off several sets of hooks in
>>> common KVM and in x86 KVM (to the vendor code).  Moving everything to
>>> vcpu_put/load() also realizes nice cleanups, especially for the Posted
>>> Interrupt code, which required some impressive mental gymnastics to
>>> understand how vCPU task migration interacted with vCPU blocking.
>>>
>>> Non-x86 folks, sorry for the noise.  I'm hoping the common parts can get
>>> applied without much fuss so that future versions can be x86-only.
>>>
>>> v2:
>>>    - Collect reviews. [Christian, David]
>>>    - Add patch to move arm64 WFI functionality out of hooks. [Marc]
>>>    - Add RISC-V to the fun.
>>>    - Add all the APICv fun.
>>
>> Have we actually followed up on the regression regarding halt_poll_ns=0 no longer disabling
>> polling for running systems?
> 
> No, I have that conversation flagged but haven't gotten back to it.  I still like
> the idea of special casing halt_poll_ns=0 to override the capability.  I can send
> a proper patch for that unless there's a different/better idea?

I think I would prefer a variant that uses the halt_poll_ns value AS IS for all
guests that have not opted in the per guest feature.
And then MAYBE have 0 as a special case to disable that also for the opted in
VMs.