Re: [PATCH v2 2/3] vfio: selftests: Align BAR mmaps for efficient IOMMU mapping

[David Matlack <dmatlack@google.com>]

On 2026-01-13 03:08 PM, Alex Mastro wrote:
> Update vfio_pci_bar_map() to align BAR mmaps for efficient huge page
> mappings. The manual mmap alignment can be removed once mmap(!MAP_FIXED)
> on vfio device fds improves to automatically return well-aligned
> addresses.

Please also mention that you added MADV_HUGEPAGE and why, and that you
dropped MAP_FILE (just mention that it was unnecessary in the first
place).

> 
> Signed-off-by: Alex Mastro <amastro@fb.com>
> ---
>  tools/testing/selftests/vfio/lib/include/libvfio.h |  9 ++++++++
>  tools/testing/selftests/vfio/lib/libvfio.c         | 25 ++++++++++++++++++++++
>  tools/testing/selftests/vfio/lib/vfio_pci_device.c | 24 ++++++++++++++++++++-
>  3 files changed, 57 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/vfio/lib/include/libvfio.h b/tools/testing/selftests/vfio/lib/include/libvfio.h
> index 279ddcd70194..5ebf8503586e 100644
> --- a/tools/testing/selftests/vfio/lib/include/libvfio.h
> +++ b/tools/testing/selftests/vfio/lib/include/libvfio.h
> @@ -23,4 +23,13 @@
>  const char *vfio_selftests_get_bdf(int *argc, char *argv[]);
>  char **vfio_selftests_get_bdfs(int *argc, char *argv[], int *nr_bdfs);
>  
> +/*
> + * Reserve virtual address space of size at an address satisfying
> + * (vaddr % align) == offset.
> + *
> + * Returns the reserved vaddr. The caller is responsible for unmapping
> + * the returned region.
> + */
> +void *mmap_aligned(size_t size, size_t align, size_t offset);

nit: Perhaps we should name this mmap_reserve()? The current name
implies something is being mmap'ed.

> +
>  #endif /* SELFTESTS_VFIO_LIB_INCLUDE_LIBVFIO_H */
> diff --git a/tools/testing/selftests/vfio/lib/libvfio.c b/tools/testing/selftests/vfio/lib/libvfio.c
> index a23a3cc5be69..4529bb1e69d1 100644
> --- a/tools/testing/selftests/vfio/lib/libvfio.c
> +++ b/tools/testing/selftests/vfio/lib/libvfio.c
> @@ -2,6 +2,9 @@
>  
>  #include <stdio.h>
>  #include <stdlib.h>
> +#include <sys/mman.h>
> +
> +#include <linux/align.h>
>  
>  #include "../../../kselftest.h"
>  #include <libvfio.h>
> @@ -76,3 +79,25 @@ const char *vfio_selftests_get_bdf(int *argc, char *argv[])
>  
>  	return vfio_selftests_get_bdfs(argc, argv, &nr_bdfs)[0];
>  }
> +
> +void *mmap_aligned(size_t size, size_t align, size_t offset)
> +{
> +	void *map_base, *map_align;
> +	size_t delta;
> +
> +	VFIO_ASSERT_GT(align, offset);
> +	delta = align - offset;
> +
> +	map_base = mmap(NULL, size + align, PROT_NONE,
> +			MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> +	VFIO_ASSERT_NE(map_base, MAP_FAILED);
> +
> +	map_align = (void *)(ALIGN((uintptr_t)map_base + delta, align) - delta);
> +
> +	if (map_align > map_base)
> +		VFIO_ASSERT_EQ(munmap(map_base, map_align - map_base), 0);
> +
> +	VFIO_ASSERT_EQ(munmap(map_align + size, map_base + align - map_align), 0);
> +
> +	return map_align;
> +}
> diff --git a/tools/testing/selftests/vfio/lib/vfio_pci_device.c b/tools/testing/selftests/vfio/lib/vfio_pci_device.c
> index 13fdb4b0b10f..03f35011b5f7 100644
> --- a/tools/testing/selftests/vfio/lib/vfio_pci_device.c
> +++ b/tools/testing/selftests/vfio/lib/vfio_pci_device.c
> @@ -12,10 +12,14 @@
>  #include <sys/mman.h>
>  
>  #include <uapi/linux/types.h>
> +#include <linux/align.h>
>  #include <linux/iommufd.h>
> +#include <linux/kernel.h>
>  #include <linux/limits.h>
> +#include <linux/log2.h>
>  #include <linux/mman.h>
>  #include <linux/overflow.h>
> +#include <linux/sizes.h>
>  #include <linux/types.h>
>  #include <linux/vfio.h>
>  
> @@ -124,20 +128,38 @@ static void vfio_pci_region_get(struct vfio_pci_device *device, int index,
>  static void vfio_pci_bar_map(struct vfio_pci_device *device, int index)
>  {
>  	struct vfio_pci_bar *bar = &device->bars[index];
> +	size_t align, size;
> +	void *vaddr;
>  	int prot = 0;

uber-nit: Put vaddr after prot to preserve the reverse-fir-tree ordering
of variables.

Here's the tip tree documentation:

  https://docs.kernel.org/process/maintainer-tip.html#variable-declarations

I should probably document somewhere that this is preferred in VFIO
selftests as well.

>  
>  	VFIO_ASSERT_LT(index, PCI_STD_NUM_BARS);
>  	VFIO_ASSERT_NULL(bar->vaddr);
>  	VFIO_ASSERT_TRUE(bar->info.flags & VFIO_REGION_INFO_FLAG_MMAP);
> +	VFIO_ASSERT_TRUE(is_power_of_2(bar->info.size));
>  
>  	if (bar->info.flags & VFIO_REGION_INFO_FLAG_READ)
>  		prot |= PROT_READ;
>  	if (bar->info.flags & VFIO_REGION_INFO_FLAG_WRITE)
>  		prot |= PROT_WRITE;
>  
> -	bar->vaddr = mmap(NULL, bar->info.size, prot, MAP_FILE | MAP_SHARED,
> +	size = bar->info.size;
> +
> +	/*
> +	 * Align BAR mmaps to improve page fault granularity during potential
> +	 * subsequent IOMMU mapping of these BAR vaddr. 1G for x86 is the
> +	 * largest hugepage size across any architecture, so no benefit from
> +	 * larger alignment. BARs smaller than 1G will be aligned by their
> +	 * power-of-two size, guaranteeing sufficient alignment for smaller
> +	 * hugepages, if present.
> +	 */
> +	align = min_t(size_t, size, SZ_1G);
> +
> +	vaddr = mmap_aligned(size, align, 0);
> +	bar->vaddr = mmap(vaddr, size, prot, MAP_SHARED | MAP_FIXED,
>  			  device->fd, bar->info.offset);
>  	VFIO_ASSERT_NE(bar->vaddr, MAP_FAILED);
> +
> +	madvise(bar->vaddr, size, MADV_HUGEPAGE);
>  }
>  
>  static void vfio_pci_bar_unmap(struct vfio_pci_device *device, int index)
> 
> -- 
> 2.47.3
>