Re: [Xen-devel] [PATCH V2] keep iommu disabled until iommu_setup() is called

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.10.12 at 00:22, Ronny Hegewald <ronny.hegewald@xxxxxxxxx> wrote:
> The iommu is enabled by default when xen is booting and later disabled in 
> iommu_setup() when no iommu is present.
> 
> But under some circumstances iommu-code can be called before iommu_setup() is 
> 
> processed. If there is no iommu available xen crashes.
> 
> This can happen for example when panic(...) is called that got introduced 
> with
> patch "x86-64: detect processors subject to AMD erratum #121 and refuse to 
> boot." since xen 4.1.3 and results in the following stacktrace:
> 
>    find_iommu_for_device
>    amd_iommu_ioapic_update_ire
>    timer_interrupt
>    enable_8259_A_irq
>    do_IRQ
>    printk_start_of_line
>    acpi_os_printf
>    io_apic_write
>    __ioapic_write_entry
>    ioapic_write_entry
>    __clear_IO_APIC_pin
>    clear_IO_APIC
>    disable_IO_APIC
>    __stop_this_cpu
>    smp_send_stop
>    machine_restart
>    panic
>    tasklet_schedule_on_cpu
>    display_cacheinfo
>    init_amd
>    generic_identify
>    identify_cpu
>    _start_xen
>    _high_start
> 
> 
> This patch fixes this by keeping the iommu disabled until iommu_setup() is 
> entered. 
> 
> Signed-off-by: Ronny Hegewald@xxxxxxxxx 

As suspected, this breaks on Intel systems: When x2APIC is pre-
enabled by the BIOS, iommu_supports_eim() now fails, causing a
panic(). I'm checking whether this can be fixed, but the interactions
are rather subtle (and there appears to be more breakage there).

Jan

> --- xen/drivers/passthrough/iommu.c.org       2012-10-05 03:38:33.000000000 
> +0000
> +++ xen/drivers/passthrough/iommu.c   2012-10-21 21:07:37.000000000 +0000
> @@ -38,7 +38,8 @@
>   *   no-intremap                Disable VT-d Interrupt Remapping
>   */
>  custom_param("iommu", parse_iommu_param);
> -bool_t __read_mostly iommu_enabled = 1;
> +bool_t iommu_enable_default __initdata = 1;
> +bool_t __read_mostly iommu_enabled;
>  bool_t __read_mostly force_iommu;
>  bool_t __initdata iommu_dom0_strict;
>  bool_t __read_mostly iommu_verbose;
> @@ -61,7 +62,7 @@
>              *ss = '\0';
>  
>          if ( !parse_bool(s) )
> -            iommu_enabled = 0;
> +            iommu_enable_default = 0;
>          else if ( !strcmp(s, "force") || !strcmp(s, "required") )
>              force_iommu = 1;
>          else if ( !strcmp(s, "workaround_bios_bug") )
> @@ -316,7 +317,7 @@
>      if ( iommu_dom0_strict )
>          iommu_passthrough = 0;
>  
> -    if ( iommu_enabled )
> +    if ( iommu_enable_default )
>      {
>          rc = iommu_hardware_setup();
>          iommu_enabled = (rc == 0);
> 
> 
> 
> 
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> Xen-devel@xxxxxxxxxxxxx 
> http://lists.xen.org/xen-devel 




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