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

[Jason Wang]

On Thu, Sep 22, 2022 at 12:12 AM 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()) {
> +            error_setg(errp, "eim=on requires support on the KVM side"
> +                             "(X2APIC_API, first shipped in v4.7)");
> +            return false;
> +        }

I wonder if we need some work on the migration compatibility here
(though it could be tricky).

Thanks

>      }
>
>      /* Currently only address widths supported are 39 and 48 bits */
> --
> 2.32.0
>