Re: [PATCH v6 4/5] intel_iommu: allow Extended Interrupt Mode when using userspace APIC

[Peter Xu <peterx@redhat.com>]

On Mon, Jul 17, 2023 at 11:29:56PM +0700, Bui Quang Minh wrote:
> On 7/17/23 17:47, Joao Martins wrote:
> > +Peter, +Jason (intel-iommu maintainer/reviewer)

Thanks for copying me, Joan.

> > 
> > On 15/07/2023 16:22, Bui Quang Minh wrote:
> > > As userspace APIC now supports x2APIC, intel interrupt remapping
> > > hardware can be set to EIM mode when userspace local APIC is used.
> > > 
> > > Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
> > > Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
> > > ---
> > >   hw/i386/intel_iommu.c | 11 -----------
> > >   1 file changed, 11 deletions(-)
> > > 
> > > diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
> > > index dcc334060c..5e576f6059 100644
> > > --- a/hw/i386/intel_iommu.c
> > > +++ b/hw/i386/intel_iommu.c
> > > @@ -4043,17 +4043,6 @@ static bool vtd_decide_config(IntelIOMMUState *s, Error **errp)
> > >                         && x86_iommu_ir_supported(x86_iommu) ?
> > >                                                 ON_OFF_AUTO_ON : ON_OFF_AUTO_OFF;
> > >       }
> > > -    if (s->intr_eim == ON_OFF_AUTO_ON && !s->buggy_eim) {
> > > -        if (!kvm_irqchip_is_split()) {
> > > -            error_setg(errp, "eim=on requires accel=kvm,kernel-irqchip=split");
> > > -            return false;
> > > -        }
> > > -        if (!kvm_enable_x2apic()) {
> > > -            error_setg(errp, "eim=on requires support on the KVM side"
> > > -                             "(X2APIC_API, first shipped in v4.7)");
> > > -            return false;
> > > -        }
> > > -    }
> > Given commit 20ca47429e ('Revert "intel_iommu: Fix irqchip / X2APIC
> > configuration checks"'), won't we regress behaviour again  for the accel=kvm
> > case by dropping the kvm_enable_x2apic() call here?
> > 
> > Perhaps if we support userspace APIC with TCG the check just needs to be redone
> > to instead avoid always requiring kvm e.g.:
> > 
> > if (kvm_irqchip_in_kernel()) {
> >      error_setg(errp, "eim=on requires accel=kvm,kernel-irqchip=split"
> >                 "(X2APIC_API, first shipped in v4.7)");
> > }
> > 
> > if (kvm_irqchip_is_split() && !kvm_enable_x2apic()) {
> >      error_setg(errp, "eim=on requires support on the KVM side"
> >                 "(X2APIC_API, first shipped in v4.7)");
> >      return false;
> > }
> 
> Thank you for your review. I think the check for kvm_irqchip_in_kernel() is
> not correct, AFAIK, kvm_irqchip_is_split() == true also means
> kvm_irqchip_in_kernel() == true on x86. To check if kernel-irqchip = on, we
> need to do something like in x86_iommu_realize
> 
>     bool irq_all_kernel = kvm_irqchip_in_kernel() &&
> !kvm_irqchip_is_split();
> 
> The original check for !kvm_irqchip_is_split means emulated/userspace APIC.
> It's because to reach that check x86_iommu_ir_supported(...) == true and
> x86_iommu_ir_supported(...) == true is not supported when kernel-irqchip =
> on (there is a check for this in x86_iommu_realize)
> 
> So I think we need to change the check to
> 
>     if (s->intr_eim == ON_OFF_AUTO_ON && !s->buggy_eim) {
>         if (kvm_irqchip_is_split() && !kvm_enable_x2apic()) {
>             error_setg(errp, "eim=on requires support on the KVM side"
>                              "(X2APIC_API, first shipped in v4.7)");
>             return false;
>         }
>     }
> 
> Is it OK?

Mostly to me, except that we may also want to keep failing if all irq chips
are in kernel?

Thanks,

-- 
Peter Xu