linux-kernel - Re: [PATCH v4 RESEND 3/5] KVM: x86: hyperv: use get_vcpu_by_vpidx() in kvm_hv_flush

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Message-ID: <87r2hs7cco.fsf@vitty.brq.redhat.com>
Date:   Mon, 17 Sep 2018 10:32:55 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Roman Kagan <rkagan@...tuozzo.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>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 RESEND 3/5] KVM: x86: hyperv: use get_vcpu_by_vpidx() in kvm_hv_flush_tlb()

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 22/08/2018 12:18, 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.
>
> ... but if it doesn't, the algorithm is now quadratic, isn't i?

Yes,

I even had an implementation with a logarythmic search back in v2 but
we had a discussion with Roman and he convinced me this is an overkill
and not currently required. It seems that with Qemu this is true indeed,
vp_index always matches vcpu is but in case some other userspace decides
to break this unwritten rule users may experience significant slowdown.

-- 
  Vitaly