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?
Best regards,
baolu
