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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 v2 2/5] KVM: x86: hyperv: introduce vp_index_to_vcpu_idx mapping
References: <20180628135313.17468-1-vkuznets@redhat.com>
        <20180628135313.17468-3-vkuznets@redhat.com>
        <20180629101134.GA15656@rkaganb.sw.ru>
        <87y3exdh2o.fsf@vitty.brq.redhat.com>
        <20180629111227.GB15656@rkaganb.sw.ru>
        <87tvplddrr.fsf@vitty.brq.redhat.com>
        <20180629125216.GC15656@rkaganb.sw.ru>
Date:   Fri, 29 Jun 2018 15:10:14 +0200
In-Reply-To: <20180629125216.GC15656@rkaganb.sw.ru> (Roman Kagan's message of
        "Fri, 29 Jun 2018 15:52:16 +0300")
Message-ID: <87h8lld9hl.fsf@vitty.brq.redhat.com>
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Roman Kagan <rkagan@virtuozzo.com> writes:

> On Fri, Jun 29, 2018 at 01:37:44PM +0200, Vitaly Kuznetsov wrote:
>> The problem we're trying to solve here is: with PV TLB flush and IPI we
>> need to walk through the supplied list of VP_INDEXes and get VCPU
>> ids. Usually they match. But in case they don't [...]
>
> Why wouldn't they *in practice*?  Only if the userspace wanted to be
> funny and assigned VP_INDEXes randomly?  I'm not sure we need to
> optimize for this case.

Can someone please remind me why we allow userspace to change it in the
first place?

>
> Note that the userspace can actually do nasty things with these
> VP_INDEXes, like, say, have them non-unique.  We need to be resilent to
> it, but don't need to optimize for it.
>
> I think I'd rather have a warning in kvm_hv_set_msr if the VP_INDEX
> being assigned is not equal to the vcpu index, and start worrying about
> optimization only if this warning starts being triggered by real
> hypervisor applications.
>
> Anyway I don't see an urgent need to bloat this patchset with optimizing
> this translation; it can be done separately, if needed.

Sure, I can use get_vcpu_by_vpidx() in the patchset pretending that is's
always fast and if we find a valid case where it is not - we'll just
optimize it later. Sounds sane to me.

-- 
  Vitaly