linux-kernel - Re: [PATCH v2 3/6] KVM: x86: make pic setup code look like ioapic setup

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Message-ID: <fc3fff2f-bcba-882f-ed92-d205dd187b10@redhat.com>
Date:   Tue, 3 Jan 2017 17:12:33 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Hildenbrand <david@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 3/6] KVM: x86: make pic setup code look like ioapic
 setup



On 03/01/2017 14:04, David Hildenbrand wrote:
> Am 16.12.2016 um 16:10 schrieb Radim Krčmář:
>> We don't treat kvm->arch.vpic specially anymore, so the setup can look
>> like ioapic.  This gets a bit more information out of return values.
>
> This originally saved us from a race condition as far as I can
> reconstruct from the commit history. Think the problem was
> vpic being set but routes not being set up yet.
> 
> commit 71ba994c94a81c37185ef2fb5190844286ba9aca
> Author: Paolo Bonzini <pbonzini@...hat.com>
> Date:   Wed Jul 29 12:31:15 2015 +0200
> 
>     KVM: x86: clean/fix memory barriers in irqchip_in_kernel
> 
>     The memory barriers are trying to protect against concurrent RCU-based
>     interrupt injection, but the IRQ routing table is not valid at the time
>     kvm->arch.vpic is written.  Fix this by writing kvm->arch.vpic last.
>     kvm_destroy_pic then need not set kvm->arch.vpic to NULL; modify it
>     to take a struct kvm_pic* and reuse it if the IOAPIC creation fails.
> 
>     Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> 
> I assume that this is now fixed via the irqchip_mode, as it is stored
> last? If so, I really like this patch :)

Yes, the previous patch referred to that when it said
"irqchip_in_kernel() tried to save a bit by reusing pic_irqchip()", and
what this commit message means by "not trteating kvm->arch.vpic
specially anymore".

Paolo