linux-kernel - Re: [PATCH v3 3/5] KVM: x86: hyperv: use get_vcpu_by_vpidx() in kvm_hv_flush

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Date:   Fri, 29 Jun 2018 19:26:36 +0300
From:   Roman Kagan <rkagan@...tuozzo.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>,
        Mohammed Gamal <mmorsy@...hat.com>,
        Cathy Avery <cavery@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/5] KVM: x86: hyperv: use get_vcpu_by_vpidx() in
 kvm_hv_flush_tlb()

On Fri, Jun 29, 2018 at 05:21:47PM +0200, Vitaly Kuznetsov wrote:
> Roman Kagan <rkagan@...tuozzo.com> writes:
> 
> > On Fri, Jun 29, 2018 at 04:14:53PM +0200, Vitaly Kuznetsov wrote:
> >> VP_INDEX almost always matches VCPU id and get_vcpu_by_vpidx() is fast,
> >> use it instead of traversing full vCPU list every time.
> >> 
> >> To support the change switch kvm_make_vcpus_request_mask() to checking
> >> vcpu_id instead of vcpu index,
> >
> > I'm afraid you can't do this: vcpu_id (== apic id) can be sparse, i.e.
> > it's not very well suited for bitmaps and can exceed the max number of
> > vcpus.
> 
> True. The bitmap should be of KVM_MAX_VCPU_ID size, not
> KVM_MAX_VCPUS.
> 
> Unfortunately there's no convenient way to get VCPU idx from VCPU
> id, kvm_vcpu_get_idx() just walks the whole list :-( I see two possible
> options:
> 1) Add vcpu_idx fields to struct kvm_vcpu
> 2) Keep the change expecting masks of KVM_MAX_VCPU_ID in
> kvm_make_vcpus_request_mask(). KVM_MAX_VCPU_ID is currently 1023 so our
> bitmaps will be 16 longs long. Not sure if it's too much.

3) rework get_vcpu_by_vpidx into get_vcpu_idx_by_vpidx followed by
get_cpu, and use the former for your purposes
4) duplicate get_vcpu_by_vpidx logic in get_vcpu_idx_by_vpidx

Roman.

P.S. I'm starting to wonder how safe this get_vcpu_* thing is WRT vcpu
removal, but that's a different story anyway.