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Message-ID: <aWfSnNIF-3xDQr4A@google.com>
Date: Wed, 14 Jan 2026 17:30:04 +0000
From: David Matlack <dmatlack@...gle.com>
To: Alex Mastro <amastro@...com>
Cc: Alex Williamson <alex@...zbot.org>, Shuah Khan <shuah@...nel.org>,
Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
kvm@...r.kernel.org, linux-kselftest@...r.kernel.org,
Jason Gunthorpe <jgg@...pe.ca>
Subject: Re: [PATCH v2 2/3] vfio: selftests: Align BAR mmaps for efficient
IOMMU mapping
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@...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
>
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