vfio: Add an IOVA bitmap support

The new facility adds a bunch of wrappers that abstract how an IOVA range
is represented in a bitmap that is granulated by a given page_size. So it
translates all the lifting of dealing with user pointers into its
corresponding kernel addresses backing said user memory into doing finally
the (non-atomic) bitmap ops to change various bits.

The formula for the bitmap is:

data[(iova / page_size) / 64] & (1ULL << (iova % 64))

Where 64 is the number of bits in a unsigned long (depending on arch)

It introduces an IOVA iterator that uses a windowing scheme to minimize the
pinning overhead, as opposed to pinning it on demand 4K at a time. Assuming
a 4K kernel page and 4K requested page size, we can use a single kernel
page to hold 512 page pointers, mapping 2M of bitmap, representing 64G of
IOVA space.

An example usage of these helpers for a given @base_iova, @page_size,
@length and __user @data:

bitmap = iova_bitmap_alloc(base_iova, page_size, length, data);
if (IS_ERR(bitmap))
return -ENOMEM;

ret = iova_bitmap_for_each(bitmap, arg, dirty_reporter_fn);

iova_bitmap_free(bitmap);

Each iteration of the @dirty_reporter_fn is called with a unique @iova
and @length argument, indicating the current range available through the
iova_bitmap. The @dirty_reporter_fn uses iova_bitmap_set() to mark dirty
areas (@iova_length) within that provided range, as following:

iova_bitmap_set(bitmap, iova, iova_length);

The facility is intended to be used for user bitmaps representing dirtied
IOVAs by IOMMU (via IOMMUFD) and PCI Devices (via vfio-pci).

Signed-off-by: Joao Martins <[e-mail]>
Signed-off-by: Yishai Hadas <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alex Williamson <[e-mail]>

drivers/vfio/Makefile

···
 # SPDX-License-Identifier: GPL-2.0
 vfio_virqfd-y := virqfd.o
 
-vfio-y += vfio_main.o
-
 obj-$(CONFIG_VFIO) += vfio.o
+
+vfio-y += vfio_main.o \
+	  iova_bitmap.o \
+
 obj-$(CONFIG_VFIO_VIRQFD) += vfio_virqfd.o
 obj-$(CONFIG_VFIO_IOMMU_TYPE1) += vfio_iommu_type1.o
 obj-$(CONFIG_VFIO_IOMMU_SPAPR_TCE) += vfio_iommu_spapr_tce.o

drivers/vfio/iova_bitmap.c

···
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2022, Oracle and/or its affiliates.
+ * Copyright (c) 2022, NVIDIA CORPORATION & AFFILIATES. All rights reserved
+ */
+#include <linux/iova_bitmap.h>
+#include <linux/mm.h>
+#include <linux/highmem.h>
+
+#define BITS_PER_PAGE (PAGE_SIZE * BITS_PER_BYTE)
+
+/*
+ * struct iova_bitmap_map - A bitmap representing an IOVA range
+ *
+ * Main data structure for tracking mapped user pages of bitmap data.
+ *
+ * For example, for something recording dirty IOVAs, it will be provided a
+ * struct iova_bitmap structure, as a general structure for iterating the
+ * total IOVA range. The struct iova_bitmap_map, though, represents the
+ * subset of said IOVA space that is pinned by its parent structure (struct
+ * iova_bitmap).
+ *
+ * The user does not need to exact location of the bits in the bitmap.
+ * From user perspective the only API available is iova_bitmap_set() which
+ * records the IOVA *range* in the bitmap by setting the corresponding
+ * bits.
+ *
+ * The bitmap is an array of u64 whereas each bit represents an IOVA of
+ * range of (1 << pgshift). Thus formula for the bitmap data to be set is:
+ *
+ *   data[(iova / page_size) / 64] & (1ULL << (iova % 64))
+ */
+struct iova_bitmap_map {
+	/* base IOVA representing bit 0 of the first page */
+	unsigned long iova;
+
+	/* page size order that each bit granules to */
+	unsigned long pgshift;
+
+	/* page offset of the first user page pinned */
+	unsigned long pgoff;
+
+	/* number of pages pinned */
+	unsigned long npages;
+
+	/* pinned pages representing the bitmap data */
+	struct page **pages;
+};
+
+/*
+ * struct iova_bitmap - The IOVA bitmap object
+ *
+ * Main data structure for iterating over the bitmap data.
+ *
+ * Abstracts the pinning work and iterates in IOVA ranges.
+ * It uses a windowing scheme and pins the bitmap in relatively
+ * big ranges e.g.
+ *
+ * The bitmap object uses one base page to store all the pinned pages
+ * pointers related to the bitmap. For sizeof(struct page*) == 8 it stores
+ * 512 struct page pointers which, if the base page size is 4K, it means
+ * 2M of bitmap data is pinned at a time. If the iova_bitmap page size is
+ * also 4K then the range window to iterate is 64G.
+ *
+ * For example iterating on a total IOVA range of 4G..128G, it will walk
+ * through this set of ranges:
+ *
+ *    4G  -  68G-1 (64G)
+ *    68G - 128G-1 (64G)
+ *
+ * An example of the APIs on how to use/iterate over the IOVA bitmap:
+ *
+ *   bitmap = iova_bitmap_alloc(iova, length, page_size, data);
+ *   if (IS_ERR(bitmap))
+ *       return PTR_ERR(bitmap);
+ *
+ *   ret = iova_bitmap_for_each(bitmap, arg, dirty_reporter_fn);
+ *
+ *   iova_bitmap_free(bitmap);
+ *
+ * Each iteration of the @dirty_reporter_fn is called with a unique @iova
+ * and @length argument, indicating the current range available through the
+ * iova_bitmap. The @dirty_reporter_fn uses iova_bitmap_set() to mark dirty
+ * areas (@iova_length) within that provided range, as following:
+ *
+ *   iova_bitmap_set(bitmap, iova, iova_length);
+ *
+ * The internals of the object uses an index @mapped_base_index that indexes
+ * which u64 word of the bitmap is mapped, up to @mapped_total_index.
+ * Those keep being incremented until @mapped_total_index is reached while
+ * mapping up to PAGE_SIZE / sizeof(struct page*) maximum of pages.
+ *
+ * The IOVA bitmap is usually located on what tracks DMA mapped ranges or
+ * some form of IOVA range tracking that co-relates to the user passed
+ * bitmap.
+ */
+struct iova_bitmap {
+	/* IOVA range representing the currently mapped bitmap data */
+	struct iova_bitmap_map mapped;
+
+	/* userspace address of the bitmap */
+	u64 __user *bitmap;
+
+	/* u64 index that @mapped points to */
+	unsigned long mapped_base_index;
+
+	/* how many u64 can we walk in total */
+	unsigned long mapped_total_index;
+
+	/* base IOVA of the whole bitmap */
+	unsigned long iova;
+
+	/* length of the IOVA range for the whole bitmap */
+	size_t length;
+};
+
+/*
+ * Converts a relative IOVA to a bitmap index.
+ * This function provides the index into the u64 array (bitmap::bitmap)
+ * for a given IOVA offset.
+ * Relative IOVA means relative to the bitmap::mapped base IOVA
+ * (stored in mapped::iova). All computations in this file are done using
+ * relative IOVAs and thus avoid an extra subtraction against mapped::iova.
+ * The user API iova_bitmap_set() always uses a regular absolute IOVAs.
+ */
+static unsigned long iova_bitmap_offset_to_index(struct iova_bitmap *bitmap,
+						 unsigned long iova)
+{
+	unsigned long pgsize = 1 << bitmap->mapped.pgshift;
+
+	return iova / (BITS_PER_TYPE(*bitmap->bitmap) * pgsize);
+}
+
+/*
+ * Converts a bitmap index to a *relative* IOVA.
+ */
+static unsigned long iova_bitmap_index_to_offset(struct iova_bitmap *bitmap,
+						 unsigned long index)
+{
+	unsigned long pgshift = bitmap->mapped.pgshift;
+
+	return (index * BITS_PER_TYPE(*bitmap->bitmap)) << pgshift;
+}
+
+/*
+ * Returns the base IOVA of the mapped range.
+ */
+static unsigned long iova_bitmap_mapped_iova(struct iova_bitmap *bitmap)
+{
+	unsigned long skip = bitmap->mapped_base_index;
+
+	return bitmap->iova + iova_bitmap_index_to_offset(bitmap, skip);
+}
+
+/*
+ * Pins the bitmap user pages for the current range window.
+ * This is internal to IOVA bitmap and called when advancing the
+ * index (@mapped_base_index) or allocating the bitmap.
+ */
+static int iova_bitmap_get(struct iova_bitmap *bitmap)
+{
+	struct iova_bitmap_map *mapped = &bitmap->mapped;
+	unsigned long npages;
+	u64 __user *addr;
+	long ret;
+
+	/*
+	 * @mapped_base_index is the index of the currently mapped u64 words
+	 * that we have access. Anything before @mapped_base_index is not
+	 * mapped. The range @mapped_base_index .. @mapped_total_index-1 is
+	 * mapped but capped at a maximum number of pages.
+	 */
+	npages = DIV_ROUND_UP((bitmap->mapped_total_index -
+			       bitmap->mapped_base_index) *
+			       sizeof(*bitmap->bitmap), PAGE_SIZE);
+
+	/*
+	 * We always cap at max number of 'struct page' a base page can fit.
+	 * This is, for example, on x86 means 2M of bitmap data max.
+	 */
+	npages = min(npages,  PAGE_SIZE / sizeof(struct page *));
+
+	/*
+	 * Bitmap address to be pinned is calculated via pointer arithmetic
+	 * with bitmap u64 word index.
+	 */
+	addr = bitmap->bitmap + bitmap->mapped_base_index;
+
+	ret = pin_user_pages_fast((unsigned long)addr, npages,
+				  FOLL_WRITE, mapped->pages);
+	if (ret <= 0)
+		return -EFAULT;
+
+	mapped->npages = (unsigned long)ret;
+	/* Base IOVA where @pages point to i.e. bit 0 of the first page */
+	mapped->iova = iova_bitmap_mapped_iova(bitmap);
+
+	/*
+	 * offset of the page where pinned pages bit 0 is located.
+	 * This handles the case where the bitmap is not PAGE_SIZE
+	 * aligned.
+	 */
+	mapped->pgoff = offset_in_page(addr);
+	return 0;
+}
+
+/*
+ * Unpins the bitmap user pages and clears @npages
+ * (un)pinning is abstracted from API user and it's done when advancing
+ * the index or freeing the bitmap.
+ */
+static void iova_bitmap_put(struct iova_bitmap *bitmap)
+{
+	struct iova_bitmap_map *mapped = &bitmap->mapped;
+
+	if (mapped->npages) {
+		unpin_user_pages(mapped->pages, mapped->npages);
+		mapped->npages = 0;
+	}
+}
+
+/**
+ * iova_bitmap_alloc() - Allocates an IOVA bitmap object
+ * @iova: Start address of the IOVA range
+ * @length: Length of the IOVA range
+ * @page_size: Page size of the IOVA bitmap. It defines what each bit
+ *             granularity represents
+ * @data: Userspace address of the bitmap
+ *
+ * Allocates an IOVA object and initializes all its fields including the
+ * first user pages of @data.
+ *
+ * Return: A pointer to a newly allocated struct iova_bitmap
+ * or ERR_PTR() on error.
+ */
+struct iova_bitmap *iova_bitmap_alloc(unsigned long iova, size_t length,
+				      unsigned long page_size, u64 __user *data)
+{
+	struct iova_bitmap_map *mapped;
+	struct iova_bitmap *bitmap;
+	int rc;
+
+	bitmap = kzalloc(sizeof(*bitmap), GFP_KERNEL);
+	if (!bitmap)
+		return ERR_PTR(-ENOMEM);
+
+	mapped = &bitmap->mapped;
+	mapped->pgshift = __ffs(page_size);
+	bitmap->bitmap = data;
+	bitmap->mapped_total_index =
+		iova_bitmap_offset_to_index(bitmap, length - 1) + 1;
+	bitmap->iova = iova;
+	bitmap->length = length;
+	mapped->iova = iova;
+	mapped->pages = (struct page **)__get_free_page(GFP_KERNEL);
+	if (!mapped->pages) {
+		rc = -ENOMEM;
+		goto err;
+	}
+
+	rc = iova_bitmap_get(bitmap);
+	if (rc)
+		goto err;
+	return bitmap;
+
+err:
+	iova_bitmap_free(bitmap);
+	return ERR_PTR(rc);
+}
+
+/**
+ * iova_bitmap_free() - Frees an IOVA bitmap object
+ * @bitmap: IOVA bitmap to free
+ *
+ * It unpins and releases pages array memory and clears any leftover
+ * state.
+ */
+void iova_bitmap_free(struct iova_bitmap *bitmap)
+{
+	struct iova_bitmap_map *mapped = &bitmap->mapped;
+
+	iova_bitmap_put(bitmap);
+
+	if (mapped->pages) {
+		free_page((unsigned long)mapped->pages);
+		mapped->pages = NULL;
+	}
+
+	kfree(bitmap);
+}
+
+/*
+ * Returns the remaining bitmap indexes from mapped_total_index to process for
+ * the currently pinned bitmap pages.
+ */
+static unsigned long iova_bitmap_mapped_remaining(struct iova_bitmap *bitmap)
+{
+	unsigned long remaining;
+
+	remaining = bitmap->mapped_total_index - bitmap->mapped_base_index;
+	remaining = min_t(unsigned long, remaining,
+	      (bitmap->mapped.npages << PAGE_SHIFT) / sizeof(*bitmap->bitmap));
+
+	return remaining;
+}
+
+/*
+ * Returns the length of the mapped IOVA range.
+ */
+static unsigned long iova_bitmap_mapped_length(struct iova_bitmap *bitmap)
+{
+	unsigned long max_iova = bitmap->iova + bitmap->length - 1;
+	unsigned long iova = iova_bitmap_mapped_iova(bitmap);
+	unsigned long remaining;
+
+	/*
+	 * iova_bitmap_mapped_remaining() returns a number of indexes which
+	 * when converted to IOVA gives us a max length that the bitmap
+	 * pinned data can cover. Afterwards, that is capped to
+	 * only cover the IOVA range in @bitmap::iova .. @bitmap::length.
+	 */
+	remaining = iova_bitmap_index_to_offset(bitmap,
+			iova_bitmap_mapped_remaining(bitmap));
+
+	if (iova + remaining - 1 > max_iova)
+		remaining -= ((iova + remaining - 1) - max_iova);
+
+	return remaining;
+}
+
+/*
+ * Returns true if there's not more data to iterate.
+ */
+static bool iova_bitmap_done(struct iova_bitmap *bitmap)
+{
+	return bitmap->mapped_base_index >= bitmap->mapped_total_index;
+}
+
+/*
+ * Advances to the next range, releases the current pinned
+ * pages and pins the next set of bitmap pages.
+ * Returns 0 on success or otherwise errno.
+ */
+static int iova_bitmap_advance(struct iova_bitmap *bitmap)
+{
+	unsigned long iova = iova_bitmap_mapped_length(bitmap) - 1;
+	unsigned long count = iova_bitmap_offset_to_index(bitmap, iova) + 1;
+
+	bitmap->mapped_base_index += count;
+
+	iova_bitmap_put(bitmap);
+	if (iova_bitmap_done(bitmap))
+		return 0;
+
+	/* When advancing the index we pin the next set of bitmap pages */
+	return iova_bitmap_get(bitmap);
+}
+
+/**
+ * iova_bitmap_for_each() - Iterates over the bitmap
+ * @bitmap: IOVA bitmap to iterate
+ * @opaque: Additional argument to pass to the callback
+ * @fn: Function that gets called for each IOVA range
+ *
+ * Helper function to iterate over bitmap data representing a portion of IOVA
+ * space. It hides the complexity of iterating bitmaps and translating the
+ * mapped bitmap user pages into IOVA ranges to process.
+ *
+ * Return: 0 on success, and an error on failure either upon
+ * iteration or when the callback returns an error.
+ */
+int iova_bitmap_for_each(struct iova_bitmap *bitmap, void *opaque,
+			 iova_bitmap_fn_t fn)
+{
+	int ret = 0;
+
+	for (; !iova_bitmap_done(bitmap) && !ret;
+	     ret = iova_bitmap_advance(bitmap)) {
+		ret = fn(bitmap, iova_bitmap_mapped_iova(bitmap),
+			 iova_bitmap_mapped_length(bitmap), opaque);
+		if (ret)
+			break;
+	}
+
+	return ret;
+}
+
+/**
+ * iova_bitmap_set() - Records an IOVA range in bitmap
+ * @bitmap: IOVA bitmap
+ * @iova: IOVA to start
+ * @length: IOVA range length
+ *
+ * Set the bits corresponding to the range [iova .. iova+length-1] in
+ * the user bitmap.
+ *
+ * Return: The number of bits set.
+ */
+void iova_bitmap_set(struct iova_bitmap *bitmap,
+		     unsigned long iova, size_t length)
+{
+	struct iova_bitmap_map *mapped = &bitmap->mapped;
+	unsigned long offset = (iova - mapped->iova) >> mapped->pgshift;
+	unsigned long nbits = max_t(unsigned long, 1, length >> mapped->pgshift);
+	unsigned long page_idx = offset / BITS_PER_PAGE;
+	unsigned long page_offset = mapped->pgoff;
+	void *kaddr;
+
+	offset = offset % BITS_PER_PAGE;
+
+	do {
+		unsigned long size = min(BITS_PER_PAGE - offset, nbits);
+
+		kaddr = kmap_local_page(mapped->pages[page_idx]);
+		bitmap_set(kaddr + page_offset, offset, size);
+		kunmap_local(kaddr);
+		page_offset = offset = 0;
+		nbits -= size;
+		page_idx++;
+	} while (nbits > 0);
+}
+EXPORT_SYMBOL_GPL(iova_bitmap_set);

include/linux/iova_bitmap.h

···
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (c) 2022, Oracle and/or its affiliates.
+ * Copyright (c) 2022, NVIDIA CORPORATION & AFFILIATES. All rights reserved
+ */
+#ifndef _IOVA_BITMAP_H_
+#define _IOVA_BITMAP_H_
+
+#include <linux/types.h>
+
+struct iova_bitmap;
+
+typedef int (*iova_bitmap_fn_t)(struct iova_bitmap *bitmap,
+				unsigned long iova, size_t length,
+				void *opaque);
+
+struct iova_bitmap *iova_bitmap_alloc(unsigned long iova, size_t length,
+				      unsigned long page_size,
+				      u64 __user *data);
+void iova_bitmap_free(struct iova_bitmap *bitmap);
+int iova_bitmap_for_each(struct iova_bitmap *bitmap, void *opaque,
+			 iova_bitmap_fn_t fn);
+void iova_bitmap_set(struct iova_bitmap *bitmap,
+		     unsigned long iova, size_t length);
+
+#endif