LGTM and completely eliminates guest VM PCI initialization slowdowns
on H100 and A100.
Also not seeing any obvious regressions on my side.
Reported-by: "Mitchell Augustin" <mitchell.augustin@xxxxxxxxxxxxx>
Reviewed-by: "Mitchell Augustin" <mitchell.augustin@xxxxxxxxxxxxx>
Tested-by: "Mitchell Augustin" <mitchell.augustin@xxxxxxxxxxxxx>
On Wed, Feb 5, 2025 at 5:18 PM Alex Williamson
<alex.williamson@xxxxxxxxxx> 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 | 24 +++++++++++++++++-------
> 1 file changed, 17 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 939920454da7..6f3e8d981311 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 *pgmask, 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;
> + *pgmask = args.pgmask;
> + }
>
> follow_pfnmap_end(&args);
> return ret;
> @@ -590,15 +592,23 @@ static int 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 pgmask;
> +
> + ret = follow_fault_pfn(vma, mm, vaddr, pfn, &pgmask,
> + 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 << PAGE_SHIFT) + ~pgmask + 1)
> + & pgmask) >> PAGE_SHIFT;
> + ret = min_t(int, npages, epfn - *pfn);
> + } else {
> ret = -EFAULT;
> + }
> }
> }
> done:
> --
> 2.47.1
>
--
Mitchell Augustin
Software Engineer - Ubuntu Partner Engineering
