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vfio/type1: Use mapping page mask for pfnmaps

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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.addr_mask.
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.

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Mitchell Augustin <mitchell.augustin@canonical.com>
Tested-by: Mitchell Augustin <mitchell.augustin@canonical.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20250218222209.1382449-7-alex.williamson@redhat.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
This commit is contained in:
Alex Williamson 2025-02-18 15:22:06 -07:00
parent 62fb8adc43
commit 0fd06844de
1 changed files with 16 additions and 7 deletions
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23
drivers/vfio/vfio_iommu_type1.c
View file

@ -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 @@ retry:
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);
} else {
ret = -EFAULT;
}
}
}
done: