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 18:47:03 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Roman Kagan <rkagan@...tuozzo.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()

Roman Kagan <rkagan@...tuozzo.com> writes:

> 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

Oh, true, thanks!

>
> 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.

As there's no CPU removal in real Hyper-V I'd expect us to find a number
of issues with Windows guests if we go down this road :-)

-- 
  Vitaly