xen/vmap: Document the vmap header

In it's current form it's rather cryptic for uninitiated, so this patch
documents everything in there to make it more palatable.

No functional change.

Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

xen/include/xen/vmap.h

@@ -1,34 +1,133 @@
+/*
+ * Interface to map physical memory onto contiguous virtual memory areas.
+ *
+ * Two ranges of linear address space are reserved for this purpose: A general
+ * purpose area (VMAP_DEFAULT) and a livepatch-specific area (VMAP_XEN). The
+ * latter is used when loading livepatches and the former for everything else.
+ */
 #if !defined(__XEN_VMAP_H__) && defined(VMAP_VIRT_START)
 #define __XEN_VMAP_H__
 
 #include <xen/mm-frame.h>
 #include <xen/page-size.h>
 
+/* Identifiers for the linear ranges tracked by vmap */
 enum vmap_region {
+    /*
+     * Region used for general purpose RW mappings. Mapping/allocating memory
+     * here can induce extra allocations for the supporting page tables.
+     */
     VMAP_DEFAULT,
+    /*
+     * Region used for loading livepatches. Can't use VMAP_DEFAULT because it
+     * must live close to the running Xen image. The caller also ensures all
+     * page tables are already in place with adequate PTE flags.
+     */
     VMAP_XEN,
+    /* Sentinel value for bounds checking */
     VMAP_REGION_NR,
 };
 
+/*
+ * Runtime initialiser for each vmap region type
+ *
+ * Must only be called once per vmap region type.
+ *
+ * @param type  Designation of the region to initialise.
+ * @param start Start address of the `type` region.
+ * @param end   End address (not inclusive) of the `type` region
+ */
 void vm_init_type(enum vmap_region type, void *start, void *end);
 
+/*
+ * Maps a set of physical ranges onto a single virtual range
+ *
+ * `mfn` is an array of `nr` physical ranges, each of which is `granularity`
+ * pages wide. `type` defines which vmap region to use for the mapping and
+ * `flags` is the PTE flags the page table leaves are meant to have.
+ *
+ * Typically used via the vmap() and vmap_contig() helpers.
+ *
+ * @param mfn          Array of mfns
+ * @param granularity  Number of contiguous pages each mfn represents
+ * @param nr           Number of mfns in the `mfn` array
+ * @param align        Alignment of the virtual area to map
+ * @param flags        PTE flags for the leaves of the PT tree.
+ * @param type         Which region to create the mappings on
+ * @return Pointer to the mapped area on success; NULL otherwise.
+ */
 void *__vmap(const mfn_t *mfn, unsigned int granularity, unsigned int nr,
              unsigned int align, unsigned int flags, enum vmap_region type);
+
+/*
+ * Map an array of pages contiguously into the VMAP_DEFAULT vmap region
+ *
+ * @param[in] mfn Pointer to the base of an array of mfns
+ * @param[in] nr  Number of mfns in the array
+ * @return Pointer to the mapped area on success; NULL otherwise.
+ */
 void *vmap(const mfn_t *mfn, unsigned int nr);
+
+/*
+ * Maps physically contiguous pages onto the VMAP_DEFAULT vmap region
+ *
+ * @param mfn Base mfn of the physical region
+ * @param nr  Number of mfns in the physical region
+ * @return Pointer to the mapped area on success; NULL otherwise.
+ */
 void *vmap_contig(mfn_t mfn, unsigned int nr);
+
+/*
+ * Unmaps a range of virtually contiguous memory from one of the vmap regions
+ *
+ * The system remembers internally how wide the mapping is and unmaps it all.
+ * It also can determine the vmap region type from the `va`.
+ *
+ * @param va Virtual base address of the range to unmap
+ */
 void vunmap(const void *va);
 
+/*
+ * Allocate `size` octets of possibly non-contiguous physical memory and map
+ * them contiguously in the VMAP_DEFAULT vmap region
+ *
+ * @param size Pointer to the base of an array of mfns
+ * @return Pointer to the mapped area on success; NULL otherwise.
+ */
 void *vmalloc(size_t size);
+
+/* Same as vmalloc(), but for the VMAP_XEN vmap region. */
 void *vmalloc_xen(size_t size);
 
+/* Same as vmalloc(), but set the contents to zero before returning */
 void *vzalloc(size_t size);
+
+/*
+ * Unmap and free memory from vmalloc(), vmalloc_xen() or vzalloc()
+ *
+ * The system remembers internally how wide the allocation is and
+ * unmaps/frees it all.
+ *
+ * @param va Virtual base address of the range to free and unmap
+ */
 void vfree(void *va);
 
+/*
+ * Analogous to vmap_contig(), but for IO memory
+ *
+ * Unlike vmap_contig(), it ensures architecturally correct cacheability
+ * settings are set for the mapped IO memory.
+ *
+ * @param pa  Physical base address of the MMIO region.
+ * @param len Length of the MMIO region in octets.
+ * @return Pointer to the mapped area on success; NULL otherwise.
+ */
 void __iomem *ioremap(paddr_t pa, size_t len);
 
 /* Return the number of pages in the mapping starting at address 'va' */
 unsigned int vmap_size(const void *va);
 
+/* Analogous to vunmap(), but for IO memory mapped via ioremap() */
 static inline void iounmap(void __iomem *va)
 {
     unsigned long addr = (unsigned long)(void __force *)va;
@@ -36,7 +135,10 @@ static inline void iounmap(void __iomem *va)
     vunmap((void *)(addr & PAGE_MASK));
 }
 
+/* Pointer to 1 octet past the end of the VMAP_DEFAULT virtual area */
 void *arch_vmap_virt_end(void);
+
+/* Initialises the VMAP_DEFAULT virtual range */
 static inline void vm_init(void)
 {
     vm_init_type(VMAP_DEFAULT, (void *)VMAP_VIRT_START, arch_vmap_virt_end());