Re: [Xen-devel] [PATCH] x86/apic: enable x2APIC mode before doing any setup

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.08.2019 15:13, Roger Pau Monne wrote:
> Current code calls apic_x2apic_probe which does some initialization
> and setup before having enabled x2APIC mode (if it's not already
> enabled by the firmware).
>
> This can lead to issues if the APIC ID doesn't match the x2APIC ID, as
> apic_x2apic_probe calls init_apic_ldr_x2apic_cluster which depending
> on the APIC mode might set cpu_2_logical_apicid using the APIC ID
> instead of the x2APIC ID (because x2APIC might not be enabled yet).
>
> Fix this by enabling x2APIC before calling apic_x2apic_probe.
>
> As a remark, this was discovered while I was trying to figure out why
> one of my test boxes didn't report any iommu faults. The root cause
> was that the iommu MSI address field was set using the stale value in
> cpu_2_logical_apicid, and thus the iommu fault interrupt would get
> lost. Even if the MSI address field gets sets to a correct value
> afterwards as soon as a single iommu fault is pending no further
> interrupts would get injected, so losing a single iommu fault
> interrupt is fatal.
>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

I'm surprised we got away with this for so long.

Jan

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