Re: [Qemu-devel] [Qemu-ppc] [PATCH 12/13] spapr/xics: ignore the lower 4K in the IRQ number space

[Greg Kurz <groug@kaod.org>]

On Wed, 13 Feb 2019 09:03:33 +0100
Cédric Le Goater <clg@kaod.org> wrote:

> On 2/13/19 2:33 AM, David Gibson wrote:
> > On Tue, Feb 12, 2019 at 08:05:53AM +0100, Cédric Le Goater wrote:  
> >> On 2/12/19 2:06 AM, David Gibson wrote:  
> >>> On Mon, Jan 07, 2019 at 07:39:45PM +0100, Cédric Le Goater wrote:  
> >>>> The IRQ number space of the XIVE and XICS interrupt mode are aligned
> >>>> when using the dual interrupt mode for the machine. This means that
> >>>> the ICS offset is set to zero in QEMU and that the KVM XICS device
> >>>> should be informed of this new value. Unfortunately, there is now way
> >>>> to do so and KVM still maintains the XICS_IRQ_BASE (0x1000) offset.
> >>>>
> >>>> Ignore the lower 4K which are not used under the XICS interrupt
> >>>> mode. These IRQ numbers are only claimed by XIVE for the CPU IPIs.
> >>>>
> >>>> Signed-off-by: Cédric Le Goater <clg@kaod.org>
> >>>> ---
> >>>>  hw/intc/xics_kvm.c | 18 ++++++++++++++++++
> >>>>  1 file changed, 18 insertions(+)
> >>>>
> >>>> diff --git a/hw/intc/xics_kvm.c b/hw/intc/xics_kvm.c
> >>>> index 651bbfdf6966..1d21ff217b82 100644
> >>>> --- a/hw/intc/xics_kvm.c
> >>>> +++ b/hw/intc/xics_kvm.c
> >>>> @@ -238,6 +238,15 @@ static void ics_get_kvm_state(ICSState *ics)
> >>>>      for (i = 0; i < ics->nr_irqs; i++) {
> >>>>          ICSIRQState *irq = &ics->irqs[i];
> >>>>  
> >>>> +        /*
> >>>> +         * The KVM XICS device considers that the IRQ numbers should
> >>>> +         * start at XICS_IRQ_BASE (0x1000). Ignore the lower 4K
> >>>> +         * numbers (only claimed by XIVE for the CPU IPIs).
> >>>> +         */
> >>>> +        if (i + ics->offset < XICS_IRQ_BASE) {
> >>>> +            continue;
> >>>> +        }
> >>>> +  
> >>>
> >>> This seems bogus to me.  The guest-visible irq numbers need to line up
> >>> between xics and xive mode, yes, but that doesn't mean we need to keep
> >>> around a great big array of unused array of ICS irq states, even in
> >>> TCG mode.  
> >>
> >> This is because the qirqs[] array is under the machine and shared between 
> >> both interrupt modes, xics and xive.  
> > 
> > I don't see how that follows.  ICSIRQState is indexed in terms of the
> > ICS source number, not the global irq number, so I don't see why it
> > has to match up with the qirq array.  
> 
> The root cause is the use of spapr->irq->nr_irqs to initialize the ICS 
> and sPAPRXive object. In case of the 'dual' backend, it covers the full 
> XIVE IRQ number space (0x2000 today) but XICS only needs 0x1000.
> 
> I think we can fix the offset issue by using the appropriate nr_irqs 
> which should be for the XICS backend : spapr->irq->nr_irqs - ics->offset
> 

Since the root cause is that the value of spapr->irq->nr_irqs should
be different in XIVE and XICS, what about fixing it during reset ?

Something like:

static void spapr_irq_reset_dual(sPAPRMachineState *spapr, Error **errp)
{
    [...]

    spapr->irq->nr_irqs = spapr_irq_current(spapr)->nr_irqs;

    spapr_irq_current(spapr)->reset(spapr, errp);
}

> 
> I keep in mind the XIVE support for nested guests and I think we will
> need to extend the IRQ number space in L1 and have the L2 use a portion
> of it (using an offset).     
> 
> C.
>  
> >>>  
> >>>>          kvm_device_access(kernel_xics_fd, KVM_DEV_XICS_GRP_SOURCES,
> >>>>                            i + ics->offset, &state, false, &error_fatal);
> >>>>  
> >>>> @@ -303,6 +312,15 @@ static int ics_set_kvm_state(ICSState *ics, int version_id)
> >>>>          ICSIRQState *irq = &ics->irqs[i];
> >>>>          int ret;
> >>>>  
> >>>> +        /*
> >>>> +         * The KVM XICS device considers that the IRQ numbers should
> >>>> +         * start at XICS_IRQ_BASE (0x1000). Ignore the lower 4K
> >>>> +         * numbers (only claimed by XIVE for the CPU IPIs).
> >>>> +         */
> >>>> +        if (i + ics->offset < XICS_IRQ_BASE) {
> >>>> +            continue;
> >>>> +        }
> >>>> +
> >>>>          state = irq->server;
> >>>>          state |= (uint64_t)(irq->saved_priority & KVM_XICS_PRIORITY_MASK)
> >>>>              << KVM_XICS_PRIORITY_SHIFT;  
> >>>  
> >>  
> >   
> 
>