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Subjectwhy Linux kernel will update BIOS's interrupt remapping table base address ? why does Linux Kernel create a new table instead of the one that BIOS provided ?
Hi all,

my questions are :
1, Linux kernel will create a new interrupt remapping table to replace
BIOS' table ?
we guys used ITP to know that BIOS' interrupt remapping table pointer
has been changed . I am tracing related code to see how linux kernel
creates interrupt remapping table.

2, why ? why linux kernel doesn't directly use BIOS' table ? our
bios has enabled x2apic mode ,and also export interrupt remapping

3, what is the relationship between interrupt remapping table with
"virtual wire mode B" , now , if firmware disabled "virtual wire mode
B" , linux kernel won't panic , otherwise , linux kernel will panic .

thanks for your replies .

from following commit messages , I have known that BIOS should provide
a interrupt remapping table and export to OS.
commit fb209bd891645bb87b9618b724f0b4928e0df3de
Author: Yinghai Lu <>
Date: Wed Dec 21 17:45:17 2011 -0800

x86, x2apic: Fallback to xapic when BIOS doesn't setup interrupt-remapping

On some of the recent Intel SNB platforms, by default bios is pre-enabling
x2apic mode in the cpu with out setting up interrupt-remapping.
This case was resulting in the kernel to panic as the cpu is already in
x2apic mode but the OS was not able to enable interrupt-remapping (which
is a pre-req for using x2apic capability).

On these platforms all the apic-ids are < 255 and the kernel can fallback to
xapic mode if the bios has not enabled interrupt-remapping (which is
mostly the case if the bios has not exported interrupt-remapping
tables to the

But now , I am carefully checking apic.c code to trace how interrupt
remapping table is enabled again by linux kernel.

struct smp_ops smp_ops define native_smp_prepare_cpus() functions ->
native_smp_prepare_cpus() -> default_setup_apic_routing() ->
enable_IR_x2apic() -> enable_IR() -> enable_intr_remapping() ->
setup_intr_remapping() -> iommu_set_intr_remapping()
I check iommu_set_intr_remapping() function , it seems that it indeed
modify INTR remapping base addr.

This function indeed follow “Intel VT for direct IO.pdf” ‘s P109
static void iommu_set_intr_remapping(struct intel_iommu *iommu, int mode)
u64 addr;
u32 sts;
unsigned long flags;

addr = virt_to_phys((void *)iommu->ir_table->base);

raw_spin_lock_irqsave(&iommu->register_lock, flags);

dmar_writeq(iommu->reg + DMAR_IRTA_REG, // #define DMAR_IRTA_REG
0xb8 /* Interrupt remapping table addr register */

/* Set interrupt-remapping table pointer */
iommu->gcmd |= DMA_GCMD_SIRTP; //P109 in Intel VT-d for direct
IO.pdf from
writel(iommu->gcmd, iommu->reg + DMAR_GCMD_REG);

readl, (sts & DMA_GSTS_IRTPS), sts);
raw_spin_unlock_irqrestore(&iommu->register_lock, flags);

* global invalidation of interrupt entry cache before enabling
* interrupt-remapping.

raw_spin_lock_irqsave(&iommu->register_lock, flags);

/* Enable interrupt-remapping */
iommu->gcmd |= DMA_GCMD_IRE;
writel(iommu->gcmd, iommu->reg + DMAR_GCMD_REG);

readl, (sts & DMA_GSTS_IRES), sts);

raw_spin_unlock_irqrestore(&iommu->register_lock, flags);
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