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Re: [Xen-devel] [PATCH v2 2/2] AMD/IOMMU: without XT, x2APIC needs to be forced into physical mode


  • To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Fri, 28 Feb 2020 14:48:56 +0000
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  • Cc: Kevin Tian <kevin.tian@xxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Fri, 28 Feb 2020 14:49:03 +0000
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  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 28/02/2020 12:12, Jan Beulich wrote:
> The wider cluster mode APIC IDs aren't generally representable. Convert
> the iommu_intremap variable into a tristate, allowing the AMD IOMMU
> driver to signal this special restriction to the apic_x2apic_probe().
> (Note: assignments to the variable get adjusted, while existing
> consumers - all assuming a boolean property - are left alone.)

I think it would be helpful to state that while we are not aware of any
hardware with this as a restriction, it is a situation which can be
created on fully x2apic-capable systems via firmware settings.

>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> v2: New.
>
> --- a/xen/arch/x86/genapic/x2apic.c
> +++ b/xen/arch/x86/genapic/x2apic.c
> @@ -236,12 +236,21 @@ const struct genapic *__init apic_x2apic
>          x2apic_phys = !iommu_intremap ||
>                        (acpi_gbl_FADT.flags & ACPI_FADT_APIC_PHYSICAL);
>      }
> -    else if ( !x2apic_phys && !iommu_intremap )
> -    {
> -        printk("WARNING: x2APIC cluster mode is not supported without 
> interrupt remapping\n"
> -               "x2APIC: forcing phys mode\n");
> -        x2apic_phys = true;
> -    }
> +    else if ( !x2apic_phys )
> +        switch ( iommu_intremap )
> +        {
> +        case iommu_intremap_off:
> +        case iommu_intremap_restricted:
> +            printk("WARNING: x2APIC cluster mode is not supported %s 
> interrupt remapping\n"
> +                   "x2APIC: forcing phys mode\n",

Any chance to fold this into a single line with "- forcing phys mode\n"
as a suffix?

> +                   iommu_intremap == iommu_intremap_off ? "without"
> +                                                        : "with restricted");
> +            x2apic_phys = true;
> +            break;
> +
> +        case iommu_intremap_full:
> +            break;
> +        }
>  
>      if ( x2apic_phys )
>          return &apic_x2apic_phys;
> --- a/xen/include/xen/iommu.h
> +++ b/xen/include/xen/iommu.h
> @@ -54,7 +54,18 @@ static inline bool_t dfn_eq(dfn_t x, dfn
>  
>  extern bool_t iommu_enable, iommu_enabled;
>  extern bool force_iommu, iommu_quarantine, iommu_verbose, iommu_igfx;
> -extern bool_t iommu_snoop, iommu_qinval, iommu_intremap, iommu_intpost;
> +extern bool_t iommu_snoop, iommu_qinval, iommu_intpost;
> +extern enum __packed iommu_intremap {
> +   /*
> +    * In order to allow traditional boolean uses of the iommu_intremap
> +    * variable, the "off" value has to come first (yielding a value of zero).
> +    */
> +   iommu_intremap_off,
> +#ifdef CONFIG_X86
> +   iommu_intremap_restricted,

This needs a note about its meaning.  How about this?

/* Interrupt remapping enabled, but only able to generate interrupts
with an 8-bit APIC ID. */

Otherwise, Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Not an issue for now, but "restricted" might become ambiguous with
future extensions.

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