Re: [Xen-devel] Unable to boot Xen 4.8 with iommu=0

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Friday, February 17, 2017 4:21 PM
> 
> I think the commit description is pretty clear. Especially this part
> 
> "This could have the nice side effect of allowing to use "iommu=off"
>  even when x2APIC was pre-enabled by the BIOS (in which case interrupt
>  remapping is a requirement, but DMA translation [obviously] isn't), but
>  that doesn't currently work (and hence x2apic_bsp_setup() forces the
>  IOMMU on rather than just interrupt remapping)."
> 
> is quite relevant in the context here. Just like Linux, we really ought
> to have a way to run with interrupt remapping, but without DMA
> remapping. So I think this old commit was a half-hearted step into
> that direction, not recognizing that it would break some other case.
> 
> So I think the only reasonable way forward to address Tamas's
> issue is to fully disentangle DMA and interrupt remapping setup,
> which I'm sure is going to take some time. As a minimal adjustment,
> though, I wonder whether that old commit's adjustment to apic.c
> shouldn't rather have set force_intremap. Tamas - could you check
> whether
> 
> --- a/xen/arch/x86/apic.c
> +++ b/xen/arch/x86/apic.c
> @@ -975,7 +975,7 @@ void __init x2apic_bsp_setup(void)
>          goto restore_out;
>      }
> 
> -    force_iommu = 1;
> +    force_intremap = 1;
> 
>      genapic = apic_x2apic_probe();
>      printk("Switched to APIC driver %s.\n", genapic->name);
> 
> makes things any better?
> 
> Jan

Looks there were some conflicting commits moving the policy 
back-and-forth. For example, your another commit in 2013 
changed back to the old way that iommu_intremap depends 
on iommu_enable:

    if ( iommu_enable )
    {
        rc = iommu_hardware_setup();
        iommu_enabled = (rc == 0);
    }
---- (added by fae03721)
    if ( !iommu_enabled )
        iommu_intremap = 0;
----
    if ( (force_iommu && !iommu_enabled) ||
         (force_intremap && !iommu_intremap) )
        panic("Couldn't enable %s and iommu=required/force",
              !iommu_enabled ? "IOMMU" : "Interrupt Remapping");

But I cannot understand why Tamas doesn't hit above panic
again...

Thanks
Kevin

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