linux-kernel - Re: [PATCH] x86: kvm: reset the bootstrap processor when it gets an INIT

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Date:	Mon, 11 Mar 2013 20:39:15 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Jan Kiszka <jan.kiszka@...mens.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"mtosatti@...hat.com" <mtosatti@...hat.com>
Subject: Re: [PATCH] x86: kvm: reset the bootstrap processor when it gets an
 INIT

On Mon, Mar 11, 2013 at 07:27:44PM +0100, Jan Kiszka wrote:
> On 2013-03-11 19:13, Gleb Natapov wrote:
> > On Mon, Mar 11, 2013 at 07:05:48PM +0100, Jan Kiszka wrote:
> >> On 2013-03-11 18:41, Gleb Natapov wrote:
> >>> On Mon, Mar 11, 2013 at 06:34:03PM +0100, Jan Kiszka wrote:
> >>>> On 2013-03-11 18:23, Gleb Natapov wrote:
> >>>>> On Mon, Mar 11, 2013 at 04:36:33PM +0100, Jan Kiszka wrote:
> >>>>>> On 2013-03-11 15:23, Paolo Bonzini wrote:
> >>>>>>> Il 11/03/2013 15:05, Gleb Natapov ha scritto:
> >>>>>>>> On Mon, Mar 11, 2013 at 03:01:40PM +0100, Jan Kiszka wrote:
> >>>>>>>>>> We are not moving away from mp_state, we are moving away from using
> >>>>>>>>>> mp_state for signaling because with nested virt INIT does not always
> >>>>>>>>>> change mp_state, not only that it can change mp_state long after signal
> >>>>>>>>>> is received after vmx off is done.
> >>>>>>>>>
> >>>>>>>>> Right.
> >>>>>>>>>
> >>>>>>>>> BTW, for that to happen, we will also need to influence the INIT level.
> >>>>>>>>> Unless I misread the spec, INIT is blocked while in root mode, and if
> >>>>>>>>> you deassert INIT before leaving root (vmxoff, vmenter), nothing
> >>>>>>>>> actually happens. So what matters is the INIT signal level at the exit
> >>>>>>>>> of root mode.
> >>>>>>>>>
> >>>>>>>> You are talking about INIT# signal received via CPU pin, right? I think
> >>>>>>>> INIT send by IPI cannot go away.
> >>>>>>>
> >>>>>>> Neither can go away.  For INIT sent by IPI, 10.4.7 says:
> >>>>>>>
> >>>>>>> Only the Pentium and P6 family processors support the INIT-deassert IPI.
> >>>>>>> An INIT-disassert IPI has no affect on the state of the APIC, other than
> >>>>>>> to reload the arbitration ID register with the value in the APIC ID
> >>>>>>> register.
> >>>>>>>
> >>>>>>> 18.27.1 also says that "In the local APIC, NMI and INIT (except for INIT
> >>>>>>> deassert) are always treated as edge triggered interrupts".
> >>>>>>>
> >>>>>>>
> >>>>>>> For INIT#, the ICH9 chipset says that "INIT# is driven low for 16 PCI
> >>>>>>> clocks" when a soft reset is requested.  So we can guess that INIT# is
> >>>>>>> also edge-triggered.
> >>>>>>
> >>>>>> Ah, ok. So, virtually, INIT stays asserted until it can be delivered in
> >>>>>> form of a reset or a vmexit.
> >>>>>>
> >>>>> vmexit clears it?
> >>>>
> >>>> It has to. Otherwise, it would hit the host on vmxoff.
> >>>>
> >>> Why do you thing this is not happening?
> >>>
> >>> Look at [1] page 10 "VMX and INIT blocking". Do you think they were
> >>> lucky to hit CPU while it was in a root mode?
> >>>
> >>> [1] http://www.invisiblethingslab.com/resources/2011/Software%20Attacks%20on%20Intel%20VT-d.pdf
> >>
> >> Interesting. And confusing. If a VMM cannot "consume" INIT events by
> >> reentering the guest nor postpone those events up to that point if they
> >> arrived in root mode, the whole vmexit-on-INIT thing is practically
> >> useless. I wonder what use case Intel had in mind while designing this.
> >>
> > I actually find it very useful. On INIT vmexit hypervisor may call
> > vmxoff and do proper reset. I find it less useful on AMD where you need
> > to send self INIT IPI, but then how you can send self SIPI?
> 
> Where's the difference? On Intel, SIPI is also not deliverable until
> after vmxoff. So that signal has to come from the INIT sender, just like
> on AMD.
> 
On Intel:
CPU 1                          CPU 2 in a guest mode
send INIT
send SIPI
                                INIT vmexit
                                vmxoff
                                reset and start from SIPI vector


On AMD;
CPU 1                           CPU 2 in a guest mode
send INIT
send SIPI
                                INIT vmexit
                                self INIT IPI
                                ???


> However, AMD allows you to NOT do a reset after leaving virtualization
> mode. On Intel, INIT is obviously irreversible, thus of limited use.
> 
Why would hypervisor send INIT to one of its CPUs if it does not want to
reset it? :)

--
			Gleb.
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