Re: [PATCH v2 4/4] intel_iommu: Fix irqchip / X2APIC configuration checks

[Peter Xu <peterx@redhat.com>]

Hi, David,

On Thu, Dec 09, 2021 at 10:08:40PM +0000, David Woodhouse wrote:
> We don't need to check kvm_enable_x2apic(). It's perfectly OK to support
> interrupt remapping even if we can't address CPUs above 254. Kind of
> pointless, but still functional.

We only checks kvm_enable_x2apic() if eim=on is set, right?  I mean, we can
still enable IR without x2apic even with current code?

Could you elaborate what's the use scenario for this patch?  Thanks in advance.

> 
> 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.
> 
> However, we do require the *split* irqchip in order to rewrite I/OAPIC
> destinations. So fix that check while we're here.
> 
> Signed-off-by: David Woodhouse <dwmw2@infradead.org>
> Reviewed-by: Peter Xu <peterx@redhat.com>
> Acked-by: Jason Wang <jasowang@redhat.com>

I think the r-b and a-b should be for patch 2 not this one? :)

> ---
>  hw/i386/intel_iommu.c | 7 +------
>  1 file changed, 1 insertion(+), 6 deletions(-)
> 
> diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
> index bd288d45bb..0d1c72f08e 100644
> --- a/hw/i386/intel_iommu.c
> +++ b/hw/i386/intel_iommu.c
> @@ -3760,15 +3760,10 @@ static bool vtd_decide_config(IntelIOMMUState *s, Error **errp)
>                                                ON_OFF_AUTO_ON : ON_OFF_AUTO_OFF;
>      }
>      if (s->intr_eim == ON_OFF_AUTO_ON && !s->buggy_eim) {
> -        if (!kvm_irqchip_in_kernel()) {
> +        if (!kvm_irqchip_is_split()) {

I think this is okay, but note that we'll already fail if !split in
x86_iommu_realize():

    bool irq_all_kernel = kvm_irqchip_in_kernel() && !kvm_irqchip_is_split();

    /* Both Intel and AMD IOMMU IR only support "kernel-irqchip={off|split}" */
    if (x86_iommu_ir_supported(x86_iommu) && irq_all_kernel) {
        error_setg(errp, "Interrupt Remapping cannot work with "
                         "kernel-irqchip=on, please use 'split|off'.");
        return;
    }

>              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;
> -        }
>      }
>  
>      /* Currently only address widths supported are 39 and 48 bits */
> -- 
> 2.31.1
> 

-- 
Peter Xu