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>

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index b92e040..3f07c37 100644 (file)
--- 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)