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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Roman Kagan <rkagan@virtuozzo.com>
Cc:     kvm@vger.kernel.org, x86@kernel.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "Michael Kelley \(EOSG\)" <Michael.H.Kelley@microsoft.com>,
        Mohammed Gamal <mmorsy@redhat.com>,
        Cathy Avery <cavery@redhat.com>,
        Wanpeng Li <wanpeng.li@hotmail.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 3/5] KVM: x86: hyperv: use get_vcpu_by_vpidx() in kvm_hv_flush_tlb()
References: <20180629141455.19525-1-vkuznets@redhat.com>
        <20180629141455.19525-4-vkuznets@redhat.com>
        <20180629150104.GE15656@rkaganb.sw.ru>
Date:   Fri, 29 Jun 2018 17:21:47 +0200
In-Reply-To: <20180629150104.GE15656@rkaganb.sw.ru> (Roman Kagan's message of
        "Fri, 29 Jun 2018 18:01:05 +0300")
Message-ID: <87a7rdd3ec.fsf@vitty.brq.redhat.com>
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Roman Kagan <rkagan@virtuozzo.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.

-- 
  Vitaly