Re: [PATCH 0/1] x2apic implementation for kvm

From: Sheng Yang <sheng@linux.intel.com>
To: Gleb Natapov <gleb@redhat.com>
Cc: kvm@vger.kernel.org
Subject: Re: [PATCH 0/1] x2apic implementation for kvm
Date: Mon, 25 May 2009 14:48:03 +0800	[thread overview]
Message-ID: <200905251448.04183.sheng@linux.intel.com> (raw)
In-Reply-To: <20090525063811.GC3948@redhat.com>

On Monday 25 May 2009 14:38:11 Gleb Natapov wrote:
> On Mon, May 25, 2009 at 02:30:05PM +0800, Sheng Yang wrote:
> > On Monday 25 May 2009 14:13:23 Gleb Natapov wrote:
> > > On Mon, May 25, 2009 at 02:08:26PM +0800, Sheng Yang wrote:
> > > > On Friday 22 May 2009 01:37:53 Gleb Natapov wrote:
> > > > > This is implementation of x2apic for KVM that I wrote a while ago.
> > > > > Unfortunately there is no guest that can take advantage of it since
> > > > > Linux doesn't (yet?) use x2apic if interrupt remapping is not
> > > > > enabled and I don't feel like implement interrupt remapping device
> > > > > :)
> > > > >
> > > > > Re-based to latest kvm tree for your viewing pleasure and feedback.
> > > >
> > > > Yeah... x2apic is for interrupt remapping, and interrupt remapping is
> > > > for VT-d engine. So if we don't want to virtualize VT-d engine and
> > > > interrupt remapping, x2apic is useless for the guest... And VT-d
> > > > engine(and related things) virtualization is far from our scope
> > > > now...
> > >
> > > Can you explain why "x2apic is for interrupt remapping"? I can
> > > understand why interrupt remapping is needed to use x2apic in certain
> > > circumstances (apic ids > 256). x2apic has better virtualizable
> > > interface and that is why we want to emulate it. Can you name one
> > > technical reason why it can't be done in a context of KVM?
> >
> > As you know, x2apic is for apic id > 256. So, at least on the real
> > machine, if there is no interrupt remapping, x2apic almost can't take
> > much advantage, because IOAPIC and MSI/MSI-X delivery still using 8bit
> > apic id, so any external interrupt can't benefit from it(though IPI can
> > benefit some, and maybe some gain from MSR rather than MMIO). And this is
> > the main purpose for x2apic, that's why Linux kernel limited x2apic using
> > with interrupt remapping.
> >
> > I am not sure the things looks like in the context of KVM. I think mostly
> > even you virtualize it, it can't replace lapic, for guest won't use it(I
> > am not sure about the Windows). Can you explain more detail?
>
> KVM will expose x2apic, but will use apic ids < 256. Linux has to be
> changed to use x2apic if it runs as a guest. Windows, when running as a
> guest, partially paravirtualize lapic by using MSRs to access frequently
> used registers. The MSR interface Windows uses is very similar to x2apic
> by the way and KVM will support it too.
>
OK, you are totally talking about PV. For PV, I think let host kernel accept 
the modification is more important here. (And for PV, using hypercall seems 
more directly).

-- 
regards
Yang, Sheng