linux-kernel - Re: [PATCH v3] enable x2APIC without interrupt remapping under KVM

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Message-ID: <m17hyt1p8s.fsf@fess.ebiederm.org>
Date:	Tue, 30 Jun 2009 12:43:47 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Gleb Natapov <gleb@...hat.com>
Cc:	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Sheng Yang <sheng@...ux.intel.com>,
	"kvm\@vger.kernel.org" <kvm@...r.kernel.org>,
	"avi\@redhat.com" <avi@...hat.com>
Subject: Re: [PATCH v3] enable x2APIC without interrupt remapping under KVM

Gleb Natapov <gleb@...hat.com> writes:

> Considering that interrupt remapping is fairly new feature
> are you saying that hotplug (pci and cpu) on x86 is a horrible
> hack on Linux?

Just the current cpu hotplug path.

>> It is better than nothing but it certainly is not something
>> I would expect to work all of the time.
>> 
> Because of horrible code or non-complaint ioapic implementation out
> there? If later this is not a big issue for KVM.

Both.  And even in spec ioapics can't be made to work 100% reliably.

>> Interrupt remapping is the one case where we have hardware
>> that works according to spec and that works reasonably well.
>> 
> I am sure when there was only one ioapic implementation in existence
> it worked according to a spec (and if not spec was changed :)) Give
> interrupt remapping some time and than we will see how above statement
> holds.

What we need for cpu hotplug/unplug is on the tested path now.
That is a significant difference.

Eric
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