linux-kernel - Re: [PATCH v2 1/2] KVM: X86: Add per-VM no-HLT-exiting capability

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Date:   Wed, 14 Feb 2018 11:26:57 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v2 1/2] KVM: X86: Add per-VM no-HLT-exiting capability

2018-02-14 0:02 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
> On 05/02/2018 07:57, Wanpeng Li wrote:
>> From: Wanpeng Li <wanpengli@...cent.com>
>>
>> If host CPUs are dedicated to a VM, we can avoid VM exits on HLT.
>> This patch adds the per-VM non-HLT-exiting capability.
>>
>> Cc: Paolo Bonzini <pbonzini@...hat.com>
>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
>> ---
>> v1 -> v2:
>>  * vmx_clear_hlt() around INIT handling
>>  * vmx_clear_hlt() upon SMI and implement auto halt restart
>
> Hi Wanpeng,
>
> sorry I could not answer before.
>
> We do not need to implement AutoHalt.  It's a messy functionality and
> the way it works is much simpler: on RSM the microcode reads AutoHALT's
> bit 0 and... decrements RIP if it is 1.  All you need to do however is
> clear the activity state.  Guests should expect anyway that "CLI;HLT"
> can be interrupted by an NMI and follow it with a JMP.

Thanks for pointing out.

>
> Second, I would prefer to implement at the same time MWAIT and PAUSE
> passthrough, as in https://www.spinics.net/lists/kvm/msg159517.html:

Understand.

>
>> The three capabilities are more or less all doing the same thing.
>> Perhaps it would make some sense to only leave PAUSE spin loops in
>> guest, but not HLT/MWAIT; but apart from that I think users would
>> probably enable all of them.  So I think we should put in the
>> documentation that blindly passing the KVM_CHECK_EXTENSION result to
>> KVM_ENABLE_CAP is a valid thing to do when vCPUs are associated to
>> dedicated physical CPUs.
>>
>> Let's get rid of KVM_CAP_X86_GUEST_MWAIT altogether and
>> add a new capability.  But let's use just one.
>
> Thanks again for your work, and sorry for slightly contradicting Radim's
> review.  I've rebased and applied patch 2.

No problem. You and Radim's review is always appreciated and helpful.

Regards,
Wanpeng Li