[Xen-changelog] [xen stable-4.12] x86/apic: enable x2APIC mode before doing any setup

[patchbot@xxxxxxx]

commit a5fc5536fb7c72827dce1ee135ee51af7aac2b79
Author:     Roger Pau Monné <roger.pau@xxxxxxxxxx>
AuthorDate: Mon Sep 23 14:17:46 2019 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Sep 23 14:17:46 2019 +0200

    x86/apic: enable x2APIC mode before doing any setup
    
    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>
    master commit: 260940578de348c38f18cadc6fa53f499e57919c
    master date: 2019-08-07 12:09:51 +0200
---
 xen/arch/x86/apic.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/apic.c b/xen/arch/x86/apic.c
index 2a2432619e..716cfbcb5d 100644
--- a/xen/arch/x86/apic.c
+++ b/xen/arch/x86/apic.c
@@ -946,15 +946,15 @@ void __init x2apic_bsp_setup(void)
 
     force_iommu = 1;
 
-    genapic = *apic_x2apic_probe();
-    printk("Switched to APIC driver %s.\n", genapic.name);
-
     if ( !x2apic_enabled )
     {
         x2apic_enabled = true;
         __enable_x2apic();
     }
 
+    genapic = *apic_x2apic_probe();
+    printk("Switched to APIC driver %s\n", genapic.name);
+
 restore_out:
     restore_IO_APIC_setup(ioapic_entries);
     unmask_8259A();
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.12

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