linux-kernel - Re: [PATCH v6 8/8] kvm: vmx: virtualize split lock detection

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Date:   Thu, 26 Mar 2020 12:10:18 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Xiaoyao Li <xiaoyao.li@...el.com>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>, hpa@...or.com,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Tony Luck <tony.luck@...el.com>
Subject: Re: [PATCH v6 8/8] kvm: vmx: virtualize split lock detection

Xiaoyao Li <xiaoyao.li@...el.com> writes:
> On 3/25/2020 8:40 AM, Thomas Gleixner wrote:
>> Xiaoyao Li <xiaoyao.li@...el.com> writes:
>>>   static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>>>   {
>>>   	struct vcpu_vmx *vmx = to_vmx(vcpu);
>>> @@ -4725,12 +4746,13 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>>>   	case AC_VECTOR:
>>>   		/*
>>>   		 * Reflect #AC to the guest if it's expecting the #AC, i.e. has
>>> -		 * legacy alignment check enabled.  Pre-check host split lock
>>> -		 * support to avoid the VMREADs needed to check legacy #AC,
>>> -		 * i.e. reflect the #AC if the only possible source is legacy
>>> -		 * alignment checks.
>>> +		 * legacy alignment check enabled or split lock detect enabled.
>>> +		 * Pre-check host split lock support to avoid further check of
>>> +		 * guest, i.e. reflect the #AC if host doesn't enable split lock
>>> +		 * detection.
>>>   		 */
>>>   		if (!split_lock_detect_on() ||
>>> +		    guest_cpu_split_lock_detect_on(vmx) ||
>>>   		    guest_cpu_alignment_check_enabled(vcpu)) {
>> 
>> If the host has split lock detection disabled then how is the guest
>> supposed to have it enabled in the first place?
>> 
> It is ||

Again. If the host has it disabled, then the feature flag is OFF. So
how is the hypervisor exposing it in the first place?

Thanks,

        tglx