Re: [Xen-devel] [PATCH v3] IOMMU: keep disabled until iommu_setup() is called

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.10.12 at 17:49, Jan Beulich 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 as introduced
> with the patch "x86-64: detect processors subject to AMD erratum #121
> and refuse to boot" since xen 4.1.3, resulting in
> find_iommu_for_device() to be called in the context of
> disable_IO_APIC() / __stop_this_cpu().
> 
> This patch fixes this by keeping the iommu disabled until iommu_setup()
> is entered.
> 
> Originally-by: Ronny Hegewald <ronny.hegewald@xxxxxxxxx>
> 
> In order for iommu_enable to be off initially, iommu_supports_eim()
> must not depend on it anymore, nor must acpi_parse_dmar(). The former
> in turn requires that iommu_intremap gets uncoupled from iommu_enabled
> (in particular, failure during IOMMU setup should no longer result in
> iommu_intremap getting cleared by generic code; IOMMU specific code
> can still do so provided in can live with the consequences).
> 
> 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).
> 
> For consistency with VT-d, make the AMD IOMMU code also skip all ACPI
> table parsing when neither iommu_enable nor iommu_intremap are set.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

So after having got acks from both Intel and AMD, I committed
this to -unstable, but I'm rather hesitant to backport this onto the
4.x branches (not the least because in the end this is just
cosmetic for the problem you reported - the box would not boot
in either case, just that with the patch it is more obvious why).

Jan


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