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Date:   Fri, 29 Jun 2018 18:55:32 +0300
From:   Roman Kagan <rkagan@virtuozzo.com>
To:     Vitaly Kuznetsov <vkuznets@redhat.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
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        Wanpeng Li <wanpeng.li@hotmail.com>, linux-kernel@vger.kernel.org
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On Fri, Jun 29, 2018 at 05:25:56PM +0200, Vitaly Kuznetsov wrote:
> Roman Kagan <rkagan@virtuozzo.com> writes:
> 
> > On Fri, Jun 29, 2018 at 03:10:14PM +0200, Vitaly Kuznetsov wrote:
> >> 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?
> >
> > I can ;)
> >
> > We used not to, and reported KVM's vcpu index as the VP_INDEX.  However,
> > later we realized that VP_INDEX needed to be persistent across
> > migrations and otherwise also known to userspace.  Relying on the kernel
> > to always initialize its indices in the same order was unacceptable, and
> > we came up with no better way of synchronizing VP_INDEX between the
> > userspace and the kernel than to let the former to set it explicitly.
> >
> > However, this is basically a future-proofing feature; in practice, both
> > QEMU and KVM initialize their indices in the same order.
> 
> 
> Thanks!
> 
> But in the theoretical case when these indices start to differ after
> migration, users will notice a slowdown which will be hard to explain,
> right?

That's exactly why I suggested a warning on VP_INDEX != vcpu index in
kvm_hv_set_msr.

> Does it justify the need for vp_idx_to_vcpu_idx?

I'd personally prefer being pointed at a scenario where this becomes
relevant first.

Roman.