[PATCH] fix x2apic defect that Linux kernel doesn't mask 8259A interrupt during the time window between changing VT-d table base address and initializing these VT-d entries

[Zhang, Lin-Bao (ESSN-MCXS-Linux Kernel R&D)]

Hi all, 
This defect can be observed when the x2apic setting in BIOS is set to "auto" 
and the BIOS has virtual wire mode enabled on a power up. This defect was found 
on a 2.6.32 based kernel.

The kernel code (smpboot.c, apic.c) does not mask 8259A interrupts before 
changing and initializing the new VT-d table when x2apic virtual wire mode is 
enable on power up. The Linux Kernel expects virtual wire mode to be disabled 
when booting and enables it when interrupts are masked.

The BIOS code builds a simple VT-d table on power up. While the Linux Kernel 
boots, it first builds an empty VT-d table and use it. After some time, the 
Linux Kernel then initializes the IO-APIC redirect table, and then initializes 
the VT-d entries. The window between initializing the redirect table and the 
VT-d entries, the 8259A interrupts are not masked. If an interrupt occurs in 
this window, the Linux Kernel will not find a valid entry for this interrupt. 
The kernel treats it to be a fatal error and panics. If the error never gets 
cleared, the Linux kernel continuously print this error:
"NMI: IOCK error (debug interrupt?) for reason"

The fix to this defect, the code change is to mask 8259A interrupts before 
changing VT-d table and initializing VT-d entries. Then unmask interrupts after 
completing the redirect table entries.


Signed-off-by: Zhang, Lin-Bao <linbao.zh...@hp.com>
Tested-by: Nigel Croxon <nigel.cro...@hp.com>

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c index 
24deb30..299172c 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -1556,7 +1556,6 @@ void __init enable_IR_x2apic(void)
        }

        local_irq_save(flags);
-       legacy_pic->mask_all();
        mask_ioapic_entries();

        if (x2apic_preenabled && nox2apic) @@ -1603,7 +1602,6 @@ void __init 
enable_IR_x2apic(void)
 skip_x2apic:
        if (ret < 0) /* IR enabling failed */
                restore_ioapic_entries();
-       legacy_pic->restore_mask();
        local_irq_restore(flags);
 }

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c index 
7c5a8c3..95fee01 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1000,7 +1000,7 @@ void __init native_smp_prepare_cpus(unsigned int max_cpus)
                zalloc_cpumask_var(&per_cpu(cpu_llc_shared_map, i), GFP_KERNEL);
        }
        set_cpu_sibling_map(0);
-
+       mask_8259A();

        if (smp_sanity_check(max_cpus) < 0) {
                pr_info("SMP disabled\n"); @@ -1037,6 +1037,8 @@ void __init 
native_smp_prepare_cpus(unsigned int max_cpus)
                apic->setup_portio_remap();

        smpboot_setup_io_apic();
+       unmask_8259A();
+
        /*
         * Set up local APIC timer on boot CPU.
         */



-- Bob(Zhang LinBao)
子曰：”不患人知不己知,患不知人也”
"If not us, who ? if not now, when ?"
ESSN-MCBS linux kernel enginner


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