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 Krčmář" <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
Date: Fri, 29 Jun 2018 18:55:32 +0300 [thread overview]
Message-ID: <20180629155532.GF15656@rkaganb.sw.ru> (raw)
In-Reply-To: <878t6xd37f.fsf@vitty.brq.redhat.com>
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.
next prev parent reply other threads:[~2018-06-29 15:56 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-06-28 13:53 [PATCH v2 0/5] KVM: x86: hyperv: PV IPI support for Windows guests Vitaly Kuznetsov
2018-06-28 13:53 ` [PATCH v2 1/5] KVM: x86: hyperv: optimize 'all cpus' case in kvm_hv_flush_tlb() Vitaly Kuznetsov
2018-06-28 13:53 ` [PATCH v2 2/5] KVM: x86: hyperv: introduce vp_index_to_vcpu_idx mapping Vitaly Kuznetsov
2018-06-28 18:49 ` kbuild test robot
2018-06-28 19:09 ` [PATCH] KVM: x86: hyperv: fix semicolon.cocci warnings kbuild test robot
2018-06-28 19:09 ` [PATCH v2 2/5] KVM: x86: hyperv: introduce vp_index_to_vcpu_idx mapping kbuild test robot
2018-06-29 10:11 ` Roman Kagan
2018-06-29 10:26 ` Vitaly Kuznetsov
2018-06-29 11:12 ` Roman Kagan
2018-06-29 11:37 ` Vitaly Kuznetsov
2018-06-29 12:52 ` Roman Kagan
2018-06-29 13:10 ` Vitaly Kuznetsov
2018-06-29 14:32 ` Roman Kagan
2018-06-29 15:25 ` Vitaly Kuznetsov
2018-06-29 15:55 ` Roman Kagan [this message]
2018-06-28 13:53 ` [PATCH v2 3/5] KVM: x86: hyperv: use vp_idx_to_vcpu_idx() in kvm_hv_flush_tlb() Vitaly Kuznetsov
2018-06-28 13:53 ` [PATCH v2 4/5] x86/hyper-v: rename ipi_arg_{ex,non_ex} structures Vitaly Kuznetsov
2018-06-28 13:53 ` [PATCH v2 5/5] KVM: x86: hyperv: implement PV IPI send hypercalls Vitaly Kuznetsov
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