On Mon, Sep 25, 2023 at 08:04:39AM +0000, Shameerali Kolothum Thodi wrote: > From: Oliver Upton [mailto:oliver.upton@xxxxxxxxx] > > On Fri, Sep 22, 2023 at 04:24:11PM +0100, Catalin Marinas wrote: > > > I was wondering if this interferes with the OS dirty tracking (not the > > > KVM one) but I think that's ok, at least at this point, since the PTE is > > > already writeable and a fault would have marked the underlying page as > > > dirty (user_mem_abort() -> kvm_set_pfn_dirty()). > > > > > > I'm not particularly fond of relying on this but I need to see how it > > > fits with the rest of the series. IIRC KVM doesn't go around and make > > > Stage 2 PTEs read-only but rather unmaps them when it changes the > > > permission of the corresponding Stage 1 VMM mapping. > > > > > > My personal preference would be to track dirty/clean properly as we do > > > for stage 1 (e.g. DBM means writeable PTE) but it has some downsides > > > like the try_to_unmap() code having to retrieve the dirty state via > > > notifiers. > > > > KVM's usage of DBM is complicated by the fact that the dirty log > > interface w/ userspace is at PTE granularity. We only want the page > > table walker to relax PTEs, but take faults on hugepages so we can do > > page splitting. Thanks for the clarification. > > > > @@ -952,6 +990,11 @@ static int stage2_map_walker_try_leaf(const struct kvm_pgtable_visit_ctx *ctx, > > > > stage2_pte_executable(new)) > > > > mm_ops->icache_inval_pou(kvm_pte_follow(new, mm_ops), granule); > > > > > > > > + /* Save the possible hardware dirty info */ > > > > + if ((ctx->level == KVM_PGTABLE_MAX_LEVELS - 1) && > > > > + stage2_pte_writeable(ctx->old)) > > > > + mark_page_dirty(kvm_s2_mmu_to_kvm(pgt->mmu), ctx->addr >> PAGE_SHIFT); > > > > + > > > > stage2_make_pte(ctx, new); > > > > > > Isn't this racy and potentially losing the dirty state? Or is the 'new' > > > value guaranteed to have the S2AP[1] bit? For stage 1 we normally make > > > the page genuinely read-only (clearing DBM) in a cmpxchg loop to > > > preserve the dirty state (see ptep_set_wrprotect()). > > > > stage2_try_break_pte() a few lines up does a cmpxchg() and full > > break-before-make, so at this point there shouldn't be a race with > > either software or hardware table walkers. Ah, I missed this. Also it was unrelated to this patch (or rather not introduced by this patch). > > In both cases the 'old' translation should have DBM cleared. Even if the > > PTE were dirty, this is wasted work since we need to do a final scan of > > the stage-2 when userspace collects the dirty log. > > > > Am I missing something? > > I think we can get rid of the above mark_page_dirty(). I will test it to confirm > we are not missing anything here. Is this the case for the other places of mark_page_dirty() in your patches? If stage2_pte_writeable() is true, it must have been made writeable earlier by a fault and the underlying page marked as dirty. -- Catalin
