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Subject: Re: [PATCH] KVM: lapic: stop advertising DIRECTED_EOI when in-kernel
 IOAPIC is in use
To:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Peter Xu <peterx@redhat.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org, x86@kernel.org,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>
References: <20180209130133.28387-1-vkuznets@redhat.com>
 <20180211052539.GA31261@xz-mi> <87r2pqxx6r.fsf@vitty.brq.redhat.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <4d3e535c-3f26-0a7e-efd7-fd55fd4928e8@redhat.com>
Date:   Mon, 12 Feb 2018 12:31:27 +0100
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On 12/02/2018 12:30, Vitaly Kuznetsov wrote:
>> - Radim's suggestion was to disable DIRECTED_EOI unconditionally but I'm not
>>   that radical :-) In theory, we may have multiple IOAPICs in userspace in
>>   future and DIRECTED_EOI can be leveraged.
> 
> I sort of agree on this, especially considering that we already have
> IOAPIC version 0x20 support in QEMU already.

Yeah, and directed EOI is faster AFAIU.

> My thoughts were: with in-kernel IOAPIC nobody does EOI broadcast
> disabling today or he's already broken -- there's no way to EOI
> level-triggered interrupts. And the patch changes nothing for QEMU
> ioapic implementation.
> 
> What I'm not sure about is if we allow migration between in-kernel and
> QEMU ioapic implementations. I don't think so but in case we do it is
> again already broken because going down from 0x20 to 0x11 will leave you
> without EOI register. Migrating from 0x11 to 0x20 should be fine, the
> guest will continue without knowing there's a new feature available.

No, we don't allow that.

Paolo