Re: [PATCH] KVM: Don't mistreat edge-triggered INIT IPI as INIT de-assert. (LAPIC)

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Jan 12, 2012 at 06:07:51PM +0100, Julian Stecklina wrote:
> Am Freitag, den 23.12.2011, 08:40 -0200 schrieb Marcelo Tosatti:
> > On Mon, Dec 19, 2011 at 02:14:27AM +0100, Julian Stecklina wrote:
> > > If the guest programs an IPI with level=0 (de-assert) and trig_mode=0 (edge),
> > > it is erroneously treated as INIT de-assert and ignored, but to quote the
> > > spec: "For this delivery mode [INIT de-assert], the level flag must be set to
> > > 0 and trigger mode flag to 1."
> > 
> > Yes, the implementation ignores INIT de-assert. Quoting the spec:
> > 
> > "(INIT Level De-assert) (Not supported in the Pentium 4 and Intel Xeon
> > processors.)"
> > 
> > Your patch below is not improving the implementation to be closer to the
> > spec: it'll trigger the INIT state initialization with trig_mode == 0
> > (which is not in accordance with your spec quote above).
> 
> After reading the spec again and consulting with the guy who wrote the
> code triggering this, it seems the whole "if (level)" in the code path
> below is superfluous. 

No. Look at whats inside "if (level)": the mp_state assignment is the
internal implementation of "delivers an INIT request to the target
processor".

According to the spec, the INIT level de-assert 

"Sends a synchronization message to all the local APICs in the system
to set their arbitration IDs (stored in their Arb ID registers) to the
values of their APIC IDs (see Section 10.7, “System and APIC Bus
Arbitration”)."

So if you remove the "if (level)" check, INIT de-assert will be emulated
as INIT!

> Regarding level my spec says:
> 
> "This flag has no meaning in Pentium 4 and Intel Xeon processors, and
> will always be issued as a 1."
> 
> Judging from Section 10.2 this means every CPU where the LAPICs
> communicate over the system bus (everything after and including the P4).
> Thus the if can be removed, since its condition is always true
> (regardless of what the programmer actually programmed into the level
> field).
> 
> If there is no objection, I'll submit a revised patch.
> 
> > > Signed-off-by: Julian Stecklina <js@alien8.de>
> > > ---
> > >  arch/x86/kvm/lapic.c |    2 +-
> > >  1 files changed, 1 insertions(+), 1 deletions(-)
> > > 
> > > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > > index a7f3e65..260770d 100644
> > > --- a/arch/x86/kvm/lapic.c
> > > +++ b/arch/x86/kvm/lapic.c
> > > @@ -433,7 +433,7 @@ static int __apic_accept_irq(struct kvm_lapic *apic, int delivery_mode,
> > >  		break;
> > >  
> > >  	case APIC_DM_INIT:
> > > -		if (level) {
> > > +		if (!trig_mode || level) {
> > >  			result = 1;
> > >  			vcpu->arch.mp_state = KVM_MP_STATE_INIT_RECEIVED;
> > >  			kvm_make_request(KVM_REQ_EVENT, vcpu);
> > > -- 
> > > 1.7.7.4
> > > 
> 
> Regards, Julian
>