linux-kernel - Re: [PATCH 2/3] KVM: X86: implement the logic for spinlock optimization

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Date:   Mon, 7 Aug 2017 20:28:13 +0800
From:   "Longpeng(Mike)" <longpeng.mike@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     "Longpeng(Mike)" <longpeng2@...wei.com>, rkrcmar@...hat.com,
        agraf@...e.com, borntraeger@...ibm.com, cohuck@...hat.com,
        christoffer.dall@...aro.org, marc.zyngier@....com,
        james.hogan@...tec.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, weidong.huang@...wei.com,
        arei.gonglei@...wei.com, wangxinxin.wang@...wei.com,
        david@...hat.com
Subject: Re: [PATCH 2/3] KVM: X86: implement the logic for spinlock
 optimization



On 08/07/2017 06:45 PM, Paolo Bonzini wrote:
> On 07/08/2017 10:44, Longpeng(Mike) wrote:
>> +
>> +	/*
>> +	 * Intel sdm vol3 ch-25.1.3 says: The “PAUSE-loop exiting”
>> +	 * VM-execution control is ignored if CPL > 0. So the vcpu
>> +	 * is always exiting with CPL=0 if it uses PLE.
> 
> This is not true (how can it be?).  What 25.1.3 says is, the VCPU is
> always at CPL=0 if you get a PAUSE exit (reason 40) and PAUSE exiting is
> 0 (it always is for KVM).  But here you're looking for a VCPU that
> didn't get a PAUSE exit, so the CPL can certainly be 3.
> 

Hi Paolo,

My comment above is something wrong(please forgive my poor English), my 
origin meaning is:
	The “PAUSE-loop exiting” VM-execution control is ignored if
	CPL > 0. So the vcpu's CPL is must 0 if it exits due to PLE.

* kvm_arch_spin_in_kernel() returns whether the vcpu(which exits due to 
spinlock) is CPL=0. It only be called by kvm_vcpu_on_spin(), and the 
input vcpu is 'me' which get a PAUSE exit now. *

I split kvm_arch_vcpu_in_kernel(in RFC) into two functions: 
kvm_arch_spin_in_kernel and kvm_arch_preempt_in_kernel


Because of KVM/VMX L1 never set CPU_BASED_PAUSE_EXITING and only set
SECONDARY_EXEC_PAUSE_LOOP_EXITING if supported, so for L1:
1. get a PAUSE exit with CPL=0 if PLE is supported
2. never get a PAUSE exit if don't support PLE

So, I think it can direct return true(CPL=0) if supports PLE.

But for nested KVM/VMX(I'm not familiar with nested), it could set 
CPU_BASED_PAUSE_EXITING, so I think get_cpl() is also needed.


If the above is correct, what about this way( we can save a vmcs_read 
opeartion for L1):

kvm_arch_vcpu_spin_in_kernel(vcpu)
{
	if (!is_guest_mode(vcpu))
		return true;

	return vmx_get_cpl(vcpu) == 0;
}

kvm_vcpu_on_spin()
{
	/* @me get a PAUSE exit */
	me_in_kernel = kvm_arch_vcpu_spin_in_kernel(me);
	...
	for each vcpu {
		...
		if (me_in_kernel && !...preempt_in_kernel(vcpu))
			continue;
		...
	}
	...
}

---
Regards,
Longpeng(Mike)

> However, I understand that vmx_get_cpl can be a bit slow here.  You can
> actually read SS's access rights directly in this function and get the
> DPL from there, that's going to be just a single VMREAD.
> 
> The only difference is when vmx->rmode.vm86_active=1.  However,
> pause-loop exiting is not working properly anyway if
> vmx->rmode.vm86_active=1, because CPL=3 according to the processor.
> 
> Paolo
> 
>> +	 * The following block needs less cycles than vmx_get_cpl().
>> +	 */
>> +	if (cpu_has_secondary_exec_ctrls())
>> +		secondary_exec_ctrl = vmcs_read32(SECONDARY_VM_EXEC_CONTROL);
>> +	if (secondary_exec_ctrl & SECONDARY_EXEC_PAUSE_LOOP_EXITING)
>> +		return true;
>> +
> 
> Paolo
>