x86, doc: Adding comments about .iommu_table and its neighbors.

Updating the linker section with comments about .iommu_table and
some other ones that I know of.

CC: Sam Ravnborg <sam@ravnborg.org>
CC: H. Peter Anvin <hpa@zytor.com>
CC: Fujita Tomonori <fujita.tomonori@lab.ntt.co.jp>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
LKML-Reference: <1282933173-19960-1-git-send-email-konrad.wilk@oracle.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 22 insertions, 0 deletions

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index b92e040466c1..3f07c370f761 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -242,6 +242,12 @@ SECTIONS
                 __x86_cpu_dev_end = .;
         }
 
+        /*
+         * start address and size of operations which during runtime
+         * can be patched with virtualization friendly instructions or
+         * baremetal native ones. Think page table operations.
+         * Details in paravirt_types.h
+         */
         . = ALIGN(8);
         .parainstructions : AT(ADDR(.parainstructions) - LOAD_OFFSET) {
                 __parainstructions = .;
@@ -249,6 +255,11 @@ SECTIONS
                 __parainstructions_end = .;
         }
 
+        /*
+         * struct alt_inst entries. From the header (alternative.h):
+         * "Alternative instructions for different CPU types or capabilities"
+         * Think locking instructions on spinlocks.
+         */
         . = ALIGN(8);
         .altinstructions : AT(ADDR(.altinstructions) - LOAD_OFFSET) {
                 __alt_instructions = .;
@@ -256,10 +267,21 @@ SECTIONS
                 __alt_instructions_end = .;
         }
 
+        /*
+         * And here are the replacement instructions. The linker sticks
+         * them as binary blobs. The .altinstructions has enough data to
+         * get the address and the length of them to patch the kernel safely.
+         */
         .altinstr_replacement : AT(ADDR(.altinstr_replacement) - LOAD_OFFSET) {
                 *(.altinstr_replacement)
         }
 
+        /*
+         * struct iommu_table_entry entries are injected in this section.
+         * It is an array of IOMMUs which during run time gets sorted depending
+         * on its dependency order. After rootfs_initcall is complete
+         * this section can be safely removed.
+         */
         .iommu_table : AT(ADDR(.iommu_table) - LOAD_OFFSET) {
                 __iommu_table = .;
                 *(.iommu_table)