linux-kernel - Re: [PATCH] KVM: nVMX: nested VPID emulation

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Date:	Wed, 16 Sep 2015 14:10:14 +0800
From:	Wanpeng Li <wanpeng.li@...mail.com>
To:	Jan Kiszka <jan.kiszka@...mens.com>,
	Paolo Bonzini <pbonzini@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: nVMX: nested VPID emulation

On 9/16/15 1:20 PM, Jan Kiszka wrote:
> On 2015-09-16 04:36, Wanpeng Li wrote:
>> On 9/16/15 1:32 AM, Jan Kiszka wrote:
>>> On 2015-09-15 12:14, Wanpeng Li wrote:
>>>> On 9/14/15 10:54 PM, Jan Kiszka wrote:
>>>>> Last but not least: the guest can now easily exhaust the host's pool of
>>>>> vpid by simply spawning plenty of VCPUs for L2, no? Is this acceptable
>>>>> or should there be some limit?
>>>> I reuse the value of vpid02 while vpid12 changed w/ one invvpid in v2,
>>>> and the scenario which you pointed out can be avoid.
>>> I cannot yet follow why there is no chance for L1 to consume all vpids
>>> that the host manages in that single, global bitmap by simply spawning a
>>> lot of nested VCPUs for some L2. What is enforcing L1 to call nested
>>> vmclear - apparently the only way, besides destructing nested VCPUs, to
>>> release such vpids again?
>> In v2, there is no direct mapping between vpid02 and vpid12, the vpid02
>> is per-vCPU for L0 and reused while the value of vpid12 is changed w/
>> one invvpid during nested vmentry. The vpid12 is allocated by L1 for L2,
>> so it will not influence global bitmap(for vpid01 and vpid02 allocation)
>> even if spawn a lot of nested vCPUs.
> Ah, I see, you limit allocation to one additional host-side vpid per
> VCPU, for nesting. That looks better. That also means all vpids for L2
> will be folded on that single vpid in hardware, right? So the major

Exactly.

> benefit comes from having separate vpids when switching between L1 and
> L2, in fact.

And also when L2's vCPUs not sched in/out on L1. Btw, your review of v3 
is a great appreciated. :-)

Regards,
Wanpeng Li

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