linux-kernel - Re: [PATCH] KVM: nVMX: Handle split-lock #AC exceptions that happen in L2

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Message-ID: <e0acbfa6-0f46-1c75-7453-fa604548b3c5@redhat.com>
Date:   Wed, 23 Jun 2021 14:30:37 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: nVMX: Handle split-lock #AC exceptions that happen
 in L2

On 23/06/21 04:43, Xiaoyao Li wrote:
>>
>> +        else if (is_alignment_check(intr_info) &&
>> +             !vmx_guest_inject_ac(vcpu))
>> +            return true;
> 
> Why choose to check in nested_vmx_l0_wants_exit, not in 
> nested_vmx_l1_wants_exit()?

nested_vmx_l0_wants_exit() == true means "this is a vmexit that needs 
some transformation before being injected into L1".  Instead, 
nested_vmx_l1_wants_exit() == true means "this is an event that should 
either be processed directly by L0, or cause a vmexit in L1"

Typically, nested_vmx_l1_wants_exit() checks the controls in vmcs12, 
while nested_vmx_l0_wants_exit() returns true unconditionally for most 
vmexits; for others it checks processor state, or other state set up by 
userspace with ioctls such as vcpu->guest_debug.

In this case it's *L0* that wants that vmexit, in order to either 
disable split-lock detection or inject a SIGBUS, so 
nested_vmx_l0_wants_exit() is the right one to test.

Paolo