linux-kernel - Re: [PATCH v4 3/4] KVM: async_pf: Force a nested vmexit if the injected #PF is async

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Date:   Tue, 27 Jun 2017 15:40:24 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v4 3/4] KVM: async_pf: Force a nested vmexit if the
 injected #PF is async_pf



On 22/06/2017 04:06, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@...mail.com>
> 
> Add an async_page_fault field to vcpu->arch.exception to identify an async 
> page fault, and constructs the expected vm-exit information fields. Force 
> a nested VM exit from nested_vmx_check_exception() if the injected #PF 
> is async page fault. Extending the userspace interface KVM_GET_VCPU_EVENTS
> and KVM_SET_VCPU_EVENTS for live migration.

I am not sure what would happen if new kernel (that can produce
async_page_fault=1) runs on top of old userspace (that can consume it).

I think it would be safer to make the new field "nested_apf", and only
set it if in guest_mode, like

	vcpu->arch.exception.nested_apf =
		is_guest_mode(vcpu) && fault->async_page_fault;
	if (vcpu->arch.exception.nested_apf)
		vcpu->arch.apf.nested_apf_token = fault->address;
	else
		vcpu->arch.cr2 = fault->address;

What do you think?

Paolo