Re: [PATCH] Revert "intel_iommu: Fix irqchip / X2APIC configuration checks"

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 21 Sep 2022 12:12:27 -0400
Peter Xu <peterx@redhat.com> wrote:

> It's true that when vcpus<=255 we don't require the length of 32bit APIC
> IDs.  However here since we already have EIM=ON it means the hypervisor
> will declare the VM as x2apic supported (e.g. VT-d ECAP register will have
> EIM bit 4 set), so the guest should assume the APIC IDs are 32bits width
> even if vcpus<=255.  In short, commit 77250171bdc breaks any simple cmdline
> that wants to boot a VM with >=9 but <=255 vcpus with:
> 
>   -device intel-iommu,intremap=on
> 
> For anyone who does not want to enable x2apic, we can use eim=off in the
> intel-iommu parameters to skip enabling KVM x2apic.
> 
> This partly reverts commit 77250171bdc02aee106083fd2a068147befa1a38, while
> keeping the valid bit on checking split irqchip, but revert the other change.
> 
> Cc: David Woodhouse <dwmw2@infradead.org>
> Cc: Claudio Fontana <cfontana@suse.de>
> Cc: Igor Mammedov <imammedo@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  hw/i386/intel_iommu.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
> index 05d53a1aa9..6524c2ee32 100644
> --- a/hw/i386/intel_iommu.c
> +++ b/hw/i386/intel_iommu.c
> @@ -3818,6 +3818,11 @@ static bool vtd_decide_config(IntelIOMMUState *s, Error **errp)
>              error_setg(errp, "eim=on requires accel=kvm,kernel-irqchip=split");
>              return false;
>          }
> +        if (!kvm_enable_x2apic()) {

above 'check' has side-effects
if it's supposed to be a check it would be better to use kvm_has_x2apic_api()
instead.

Also 77250171bdc says:
"
    The check on kvm_enable_x2apic() needs to happen *anyway* in order to
    allow CPUs above 254 even without an IOMMU, so allow that to happen
    elsewhere.
"

Looking for that elsewhere, it looks like commit dc89f32d92b was supposed
to take care of removed chunk, but that is not reachable because of > 255 vCPUs"

Likely 77250171bdc just exposed a bug in dc89f32d92b, where
the later removed kvm_enable_x2apic() always called (with split irqchip)
and made it called only when > 255 vCPUs.

So migration wise it looks like all version with it and less than 255 cpus
are broken.

Wait earlier c1bb5418e3, introduced that
   kvm_irqchip_is_split() && kvm_enable_x2apic()
'condition', also without any compat machinery to keep old behavior.

And before that kvm_enable_x2apic() was affecting only configuration
with intel_iommu (fb506e701e9b).

I'm not sure if anything could be salvaged here migration wise

PS:
I'd keep kvm_enable_x2apic() only in corrected x86_cpus_init()
and use kvm_has_x2apic_api() elsewhere for checks and bailing out.


> +            error_setg(errp, "eim=on requires support on the KVM side"
> +                             "(X2APIC_API, first shipped in v4.7)");
> +            return false;
> +        }



>      }
>  
>      /* Currently only address widths supported are 39 and 48 bits */