CONFIG_DMAR is not available for thedefaultarchitecture x86.
Result is shown for architecture ia64

Support for DMA Remapping Devices (EXPERIMENTAL)

modulename: dmar.ko

and/or

pci-dma.ko

configname: CONFIG_DMAR

IA-64 Linux Kernel Configuration
└─>Bus options (PCI, PCMCIA)
└─>Support for DMA Remapping Devices (EXPERIMENTAL)
In linux kernel since version 2.6.24 (release Date: 2008-01-24)  
DMA remapping (DMAR) devices support enables independent address
translations for Direct Memory Access (DMA) from devices.
These DMA remapping devices are reported via ACPI tables
and include PCI device scope covered by these DMA
remapping devices.

source code:
drivers/pci/
depends
CONFIG_ACPI
CONFIG_EXPERIMENTAL
CONFIG_IA64_GENERIC
NOT CONFIG_IA64_HP_SIM