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

[Alex Mastro <amastro@fb.com>]

On Wed, Jan 14, 2026 at 05:30:04PM +0000, David Matlack wrote:
> 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).

Ack

> 
> > 
> > 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.

SGTM

> 
> > +
> >  #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.

Ah, thanks. I usually try to do this but missed it here.

> 
> >  
> >  	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
> >