> From: Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx>
> Sent: Friday, October 20, 2023 10:22 AM
>
> On 10/19/23 7:58 PM, Joao Martins wrote:
> > On 19/10/2023 01:17, Joao Martins wrote:
> >> On 19/10/2023 00:11, Jason Gunthorpe wrote:
> >>> On Wed, Oct 18, 2023 at 09:27:08PM +0100, Joao Martins wrote:
> >>>> +static int iommu_v1_read_and_clear_dirty(struct io_pgtable_ops *ops,
> >>>> + unsigned long iova, size_t size,
> >>>> + unsigned long flags,
> >>>> + struct iommu_dirty_bitmap *dirty)
> >>>> +{
> >>>> + struct amd_io_pgtable *pgtable = io_pgtable_ops_to_data(ops);
> >>>> + unsigned long end = iova + size - 1;
> >>>> +
> >>>> + do {
> >>>> + unsigned long pgsize = 0;
> >>>> + u64 *ptep, pte;
> >>>> +
> >>>> + ptep = fetch_pte(pgtable, iova, &pgsize);
> >>>> + if (ptep)
> >>>> + pte = READ_ONCE(*ptep);
> >>> It is fine for now, but this is so slow for something that is such a
> >>> fast path. We are optimizing away a TLB invalidation but leaving
> >>> this???
> >>>
> >> More obvious reason is that I'm still working towards the 'faster' page
> table
> >> walker. Then map/unmap code needs to do similar lookups so thought of
> reusing
> >> the same functions as map/unmap initially. And improve it afterwards or
> when
> >> introducing the splitting.
> >>
> >>> It is a radix tree, you walk trees by retaining your position at each
> >>> level as you go (eg in a function per-level call chain or something)
> >>> then ++ is cheap. Re-searching the entire tree every time is madness.
> >> I'm aware -- I have an improved page-table walker for AMD[0] (not yet for
> Intel;
> >> still in the works),
> > Sigh, I realized that Intel's pfn_to_dma_pte() (main lookup function for
> > map/unmap/iova_to_phys) does something a little off when it finds a non-
> present
> > PTE. It allocates a page table to it; which is not OK in this specific case (I
> > would argue it's neither for iova_to_phys but well maybe I misunderstand
> the
> > expectation of that API).
>
> pfn_to_dma_pte() doesn't allocate page for a non-present PTE if the
> target_level parameter is set to 0. See below line 932.
>
> 913 static struct dma_pte *pfn_to_dma_pte(struct dmar_domain *domain,
> 914 unsigned long pfn, int *target_level,
> 915 gfp_t gfp)
> 916 {
>
> [...]
>
> 927 while (1) {
> 928 void *tmp_page;
> 929
> 930 offset = pfn_level_offset(pfn, level);
> 931 pte = &parent[offset];
> 932 if (!*target_level && (dma_pte_superpage(pte) ||
> !dma_pte_present(pte)))
> 933 break;
>
> So both iova_to_phys() and read_and_clear_dirty() are doing things
> right:
>
> struct dma_pte *pte;
> int level = 0;
>
> pte = pfn_to_dma_pte(dmar_domain, iova >> VTD_PAGE_SHIFT,
> &level, GFP_KERNEL);
> if (pte && dma_pte_present(pte)) {
> /* The PTE is valid, check anything you want! */
> ... ...
> }
>
> Or, I am overlooking something else?
>
This is correct.
