Re: [PATCH v2 2/5] KVM: x86: hyperv: introduce vp_index_to_vcpu_idx mapping

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 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 17:25:56 +0200	[thread overview]
Message-ID: <878t6xd37f.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <20180629143212.GD15656@rkaganb.sw.ru> (Roman Kagan's message of "Fri, 29 Jun 2018 17:32:13 +0300")

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? Does it justify the need for vp_idx_to_vcpu_idx?

In any case I sent v3 with vp_idx_to_vcpu_idx dropped for now, hope
Radim is OK with us de-coupling these discussions.

-- 
  Vitaly