On Tue, 18 Feb 2025 17:47:46 -0500
Peter Xu <peterx@xxxxxxxxxx> wrote:
> On Tue, Feb 18, 2025 at 03:22:06PM -0700, Alex Williamson wrote:
> > vfio-pci supports huge_fault for PCI MMIO BARs and will insert pud and
> > pmd mappings for well aligned mappings. follow_pfnmap_start() walks the
> > page table and therefore knows the page mask of the level where the
> > address is found and returns this through follow_pfnmap_args.pgmask.
> > Subsequent pfns from this address until the end of the mapping page are
> > necessarily consecutive. Use this information to retrieve a range of
> > pfnmap pfns in a single pass.
> >
> > With optimal mappings and alignment on systems with 1GB pud and 4KB
> > page size, this reduces iterations for DMA mapping PCI BARs by a
> > factor of 256K. In real world testing, the overhead of iterating
> > pfns for a VM DMA mapping a 32GB PCI BAR is reduced from ~1s to
> > sub-millisecond overhead.
> >
> > Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > ---
> > drivers/vfio/vfio_iommu_type1.c | 23 ++++++++++++++++-------
> > 1 file changed, 16 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > index ce661f03f139..0ac56072af9f 100644
> > --- a/drivers/vfio/vfio_iommu_type1.c
> > +++ b/drivers/vfio/vfio_iommu_type1.c
> > @@ -520,7 +520,7 @@ static void vfio_batch_fini(struct vfio_batch *batch)
> >
> > static int follow_fault_pfn(struct vm_area_struct *vma, struct mm_struct *mm,
> > unsigned long vaddr, unsigned long *pfn,
> > - bool write_fault)
> > + unsigned long *addr_mask, bool write_fault)
> > {
> > struct follow_pfnmap_args args = { .vma = vma, .address = vaddr };
> > int ret;
> > @@ -544,10 +544,12 @@ static int follow_fault_pfn(struct vm_area_struct *vma, struct mm_struct *mm,
> > return ret;
> > }
> >
> > - if (write_fault && !args.writable)
> > + if (write_fault && !args.writable) {
> > ret = -EFAULT;
> > - else
> > + } else {
> > *pfn = args.pfn;
> > + *addr_mask = args.addr_mask;
> > + }
> >
> > follow_pfnmap_end(&args);
> > return ret;
> > @@ -590,15 +592,22 @@ static long vaddr_get_pfns(struct mm_struct *mm, unsigned long vaddr,
> > vma = vma_lookup(mm, vaddr);
> >
> > if (vma && vma->vm_flags & VM_PFNMAP) {
> > - ret = follow_fault_pfn(vma, mm, vaddr, pfn, prot & IOMMU_WRITE);
> > + unsigned long addr_mask;
> > +
> > + ret = follow_fault_pfn(vma, mm, vaddr, pfn, &addr_mask,
> > + prot & IOMMU_WRITE);
> > if (ret == -EAGAIN)
> > goto retry;
> >
> > if (!ret) {
> > - if (is_invalid_reserved_pfn(*pfn))
> > - ret = 1;
> > - else
> > + if (is_invalid_reserved_pfn(*pfn)) {
> > + unsigned long epfn;
> > +
> > + epfn = (*pfn | (~addr_mask >> PAGE_SHIFT)) + 1;
> > + ret = min_t(long, npages, epfn - *pfn);
>
> s/long/unsigned long/?
ret is signed long since it's the function return and needs to be able
to return -errno, so long was the intention here. Thanks,
Alex
> Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>
>
> > + } else {
> > ret = -EFAULT;
> > + }
> > }
> > }
> > done:
> > --
> > 2.48.1
> >
>
