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

Gleb Natapov
2013-03-11T18:13:06Z

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?

> That article claims they tested it, though via MSI injection, so they
> are probably right, at least for the used CPU type.
>
> Jan
>
> --
> Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
> Corporate Competence Center Embedded Linux

--
Gleb.