Re: [PATCH 5/5] vfio-pci: Best-effort huge pfnmaps with !MAP_FIXED mappings

Peter Xu
2025-06-13T19:21:46Z

On Fri, Jun 13, 2025 at 08:09:41PM +0200, David Hildenbrand wrote:
> On 13.06.25 15:41, Peter Xu wrote:
> > This patch enables best-effort mmap() for vfio-pci bars even without
> > MAP_FIXED, so as to utilize huge pfnmaps as much as possible. It should
> > also avoid userspace changes (switching to MAP_FIXED with pre-aligned VA
> > addresses) to start enabling huge pfnmaps on VFIO bars.
> >
> > Here the trick is making sure the MMIO PFNs will be aligned with the VAs
> > allocated from mmap() when !MAP_FIXED, so that whatever returned from
> > mmap(!MAP_FIXED) of vfio-pci MMIO regions will be automatically suitable
> > for huge pfnmaps as much as possible.
> >
> > To achieve that, a custom vfio_device's get_unmapped_area() for vfio-pci
> > devices is needed.
> >
> > Note that MMIO physical addresses should normally be guaranteed to be
> > always bar-size aligned, hence the bar offset can logically be directly
> > used to do the calculation. However to make it strict and clear (rather
> > than relying on spec details), we still try to fetch the bar's physical
> > addresses from pci_dev.resource[].
> >
> > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
>
> There is likely a
>
> Co-developed-by: Alex Williamson <alex.williamson@redhat.com>
>
> missing?

Would it mean the same if we use the two SoBs like what this patch uses?
I sincerely don't know the difference.. I hope it's fine to show that this
patch was developed together. Please let me know otherwise.

>
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> > drivers/vfio/pci/vfio_pci.c | 3 ++
> > drivers/vfio/pci/vfio_pci_core.c | 65 ++++++++++++++++++++++++++++++++
> > include/linux/vfio_pci_core.h | 6 +++
> > 3 files changed, 74 insertions(+)
> >
> > diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> > index 5ba39f7623bb..d9ae6cdbea28 100644
> > --- a/drivers/vfio/pci/vfio_pci.c
> > +++ b/drivers/vfio/pci/vfio_pci.c
> > @@ -144,6 +144,9 @@ static const struct vfio_device_ops vfio_pci_ops = {
> > .detach_ioas = vfio_iommufd_physical_detach_ioas,
> > .pasid_attach_ioas = vfio_iommufd_physical_pasid_attach_ioas,
> > .pasid_detach_ioas = vfio_iommufd_physical_pasid_detach_ioas,
> > +#ifdef CONFIG_ARCH_SUPPORTS_HUGE_PFNMAP
> > + .get_unmapped_area = vfio_pci_core_get_unmapped_area,
> > +#endif
> > };
> > static int vfio_pci_probe(struct pci_dev *pdev, const struct pci_device_id *id)
> > diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> > index 6328c3a05bcd..835bc168f8b7 100644
> > --- a/drivers/vfio/pci/vfio_pci_core.c
> > +++ b/drivers/vfio/pci/vfio_pci_core.c
> > @@ -1641,6 +1641,71 @@ static unsigned long vma_to_pfn(struct vm_area_struct *vma)
> > return (pci_resource_start(vdev->pdev, index) >> PAGE_SHIFT) + pgoff;
> > }
> > +#ifdef CONFIG_ARCH_SUPPORTS_HUGE_PFNMAP
> > +/*
> > + * Hint function to provide mmap() virtual address candidate so as to be
> > + * able to map huge pfnmaps as much as possible. It is done by aligning
> > + * the VA to the PFN to be mapped in the specific bar.
> > + *
> > + * Note that this function does the minimum check on mmap() parameters to
> > + * make the PFN calculation valid only. The majority of mmap() sanity check
> > + * will be done later in mmap().
> > + */
> > +unsigned long vfio_pci_core_get_unmapped_area(struct vfio_device *device,
> > + struct file *file,
> > + unsigned long addr,
> > + unsigned long len,
> > + unsigned long pgoff,
> > + unsigned long flags)
>
> A very suboptimal way to indent this many parameters; just use two tabs at
> the beginning.

This is the default indentation from Emacs c-mode.

Since this is a VFIO file, I checked the file and looks like there's not
yet a strict rule of indentation across the whole file. I can switch to
two-tabs for sure if nobody else disagrees.

>
> > +{
> > + struct vfio_pci_core_device *vdev =
> > + container_of(device, struct vfio_pci_core_device, vdev);
> > + struct pci_dev *pdev = vdev->pdev;
> > + unsigned long ret, phys_len, req_start, phys_addr;
> > + unsigned int index;
> > +
> > + index = pgoff >> (VFIO_PCI_OFFSET_SHIFT - PAGE_SHIFT);
>
> Could do
>
> unsigned int index = pgoff >> (VFIO_PCI_OFFSET_SHIFT - PAGE_SHIFT);
>
> at the very top.

Sure.

>
> > +
> > + /* Currently, only bars 0-5 supports huge pfnmap */
> > + if (index >= VFIO_PCI_ROM_REGION_INDEX)
> > + goto fallback;
> > +
> > + /* Bar offset */
> > + req_start = (pgoff << PAGE_SHIFT) & ((1UL << VFIO_PCI_OFFSET_SHIFT) - 1);
> > + phys_len = PAGE_ALIGN(pci_resource_len(pdev, index));
> > +
> > + /*
> > + * Make sure we at least can get a valid physical address to do the
> > + * math. If this happens, it will probably fail mmap() later..
> > + */
> > + if (req_start >= phys_len)
> > + goto fallback;
> > +
> > + phys_len = MIN(phys_len, len);
> > + /* Calculate the start of physical address to be mapped */
> > + phys_addr = pci_resource_start(pdev, index) + req_start;
> > +
> > + /* Choose the alignment */
> > + if (IS_ENABLED(CONFIG_ARCH_SUPPORTS_PUD_PFNMAP) && phys_len >= PUD_SIZE) {
> > + ret = mm_get_unmapped_area_aligned(file, addr, len, phys_addr,
> > + flags, PUD_SIZE, 0);
> > + if (ret)
> > + return ret;
> > + }
> > +
> > + if (phys_len >= PMD_SIZE) {
> > + ret = mm_get_unmapped_area_aligned(file, addr, len, phys_addr,
> > + flags, PMD_SIZE, 0);
> > + if (ret)
> > + return ret;
>
> Similar to Jason, I wonder if that logic should reside in the core, and we
> only indicate the maximum page table level we support.

I replied. We can continue the discussion there.

Thanks,

--
Peter Xu