On Tue, Oct 22, 2024 at 02:08:29PM -0600, Alex Williamson wrote:
> Thanks to work by Peter Xu, support is introduced in Linux v6.12 to
> allow pfnmap insertions at PMD and PUD levels of the page table. This
> means that provided a properly aligned mmap, the vfio driver is able
> to map MMIO at significantly larger intervals than PAGE_SIZE. For
> example on x86_64 (the only architecture currently supporting huge
> pfnmaps for PUD), rather than 4KiB mappings, we can map device MMIO
> using 2MiB and even 1GiB page table entries.
>
> Typically mmap will already provide PMD aligned mappings, so devices
> with moderately sized MMIO ranges, even GPUs with standard 256MiB BARs,
> will already take advantage of this support. However in order to better
> support devices exposing multi-GiB MMIO, such as 3D accelerators or GPUs
> with resizable BARs enabled, we need to manually align the mmap.
>
> There doesn't seem to be a way for userspace to easily learn about PMD
> and PUD mapping level sizes, therefore this takes the simple approach
> to align the mapping to the power-of-two size of the region, up to 1GiB,
> which is currently the maximum alignment we care about.
>
> Cc: Peter Xu <pet...@redhat.com>
> Signed-off-by: Alex Williamson <alex.william...@redhat.com>
For the longer term, maybe QEMU can provide a function to reserve a range
of mmap with some specific alignment requirement. For example, currently
qemu_ram_mmap() does mostly the same thing (and it hides a hugetlb fix on
ppc only with 7197fb4058, which isn't a concern here). Then the complexity
can hide in that function. Kind of a comment for the future only.
Reviewed-by: Peter Xu <pet...@redhat.com>
Thanks!
> ---
> hw/vfio/helpers.c | 32 ++++++++++++++++++++++++++++++--
> 1 file changed, 30 insertions(+), 2 deletions(-)
>
> diff --git a/hw/vfio/helpers.c b/hw/vfio/helpers.c
> index b9e606e364a2..913796f437f8 100644
> --- a/hw/vfio/helpers.c
> +++ b/hw/vfio/helpers.c
> @@ -27,6 +27,7 @@
> #include "trace.h"
> #include "qapi/error.h"
> #include "qemu/error-report.h"
> +#include "qemu/units.h"
> #include "monitor/monitor.h"
>
> /*
> @@ -406,8 +407,35 @@ int vfio_region_mmap(VFIORegion *region)
> prot |= region->flags & VFIO_REGION_INFO_FLAG_WRITE ? PROT_WRITE : 0;
>
> for (i = 0; i < region->nr_mmaps; i++) {
> - region->mmaps[i].mmap = mmap(NULL, region->mmaps[i].size, prot,
> - MAP_SHARED, region->vbasedev->fd,
> + size_t align = MIN(1ULL << ctz64(region->mmaps[i].size), 1 * GiB);
> + void *map_base, *map_align;
> +
> + /*
> + * Align the mmap for more efficient mapping in the kernel. Ideally
> + * we'd know the PMD and PUD mapping sizes to use as discrete
> alignment
> + * intervals, but we don't. As of Linux v6.12, the largest PUD size
> + * supporting huge pfnmap is 1GiB (ARCH_SUPPORTS_PUD_PFNMAP is only
> set
> + * on x86_64). Align by power-of-two size, capped at 1GiB.
> + *
> + * NB. qemu_memalign() and friends actually allocate memory, whereas
> + * the region size here can exceed host memory, therefore we manually
> + * create an oversized anonymous mapping and clean it up for
> alignment.
> + */
> + map_base = mmap(0, region->mmaps[i].size + align, PROT_NONE,
> + MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> + if (map_base == MAP_FAILED) {
> + ret = -errno;
> + goto no_mmap;
> + }
> +
> + map_align = (void *)ROUND_UP((uintptr_t)map_base, (uintptr_t)align);
> + munmap(map_base, map_align - map_base);
> + munmap(map_align + region->mmaps[i].size,
> + align - (map_align - map_base));
> +
> + region->mmaps[i].mmap = mmap(map_align, region->mmaps[i].size, prot,
> + MAP_SHARED | MAP_FIXED,
> + region->vbasedev->fd,
> region->fd_offset +
> region->mmaps[i].offset);
> if (region->mmaps[i].mmap == MAP_FAILED) {
> --
> 2.46.2
>
--
Peter Xu
