linux-kernel - Re: [PATCH 5/5] KVM: nVMX: Enable nested posted interrupt processing.

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Message-ID: <54BE04F7.7030402@redhat.com>
Date:	Tue, 20 Jan 2015 08:34:15 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Wincy Van <fanwenyi0529@...il.com>
CC:	gleb@...nel.org, yang.z.zhang@...el.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Wanpeng Li <wanpeng.li@...ux.intel.com>,
	Jan Kiszka <jan.kiszka@....de>
Subject: Re: [PATCH 5/5] KVM: nVMX: Enable nested posted interrupt processing.



On 19/01/2015 13:34, Wincy Van wrote:
> 
> Actually, there is a race window between
> vmx_deliver_nested_posted_interrupt and nested_release_vmcs12
> since posted intr delivery is async:
> 
>          cpu 1
>           cpu 2
> (nested posted intr)                                      (dest vcpu,
> release vmcs12)
> vmcs12 = get_vmcs12(vcpu);
> if (!is_guest_mode(vcpu) || !vmcs12) {
>         r = -1;
>         goto out;
> }
> 
> 
> kunmap(vmx->nested.current_vmcs12_page);
> 
>        ......
> 
> 
> oops! current vmcs12 is invalid.
> 
> However, we have already checked that the destination vcpu
> is_in_guest_mode, and if L1
> want to destroy vmcs12(in handle_vmptrld/clear, etc..), the dest vcpu
> must have done a nested
> vmexit and a non-nested vmexit(handle_vmptr***).
> 
> Hence, we can disable local interrupts while delivering nested posted
> interrupts to make sure
> we are faster than the destination vcpu. This is a bit tricky but it
> an avoid that race. I think we
> do not need to add a spin lock here. RCU does not fit this case, since
> it will introduce a
> new race window between the rcu handler and handle_vmptr**.
> 
> I am wondering that whether there is a better way : )

Why not just use a spinlock?

Paolo
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