On 20/10/2023 03:21, Baolu Lu wrote: > 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? You're right, thanks for the keeping me straight -- I was already doing the right thing. I've forgotten about it in the midst of the other code -- Probably worth a comment in the caller to make it obvious. Joao