> -----Original Message----- > From: Catalin Marinas [mailto:catalin.marinas@xxxxxxx] > Sent: 26 September 2023 16:20 > To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@xxxxxxxxxx> > Cc: Oliver Upton <oliver.upton@xxxxxxxxx>; kvmarm@xxxxxxxxxxxxxxx; > kvm@xxxxxxxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; maz@xxxxxxxxxx; > will@xxxxxxxxxx; james.morse@xxxxxxx; suzuki.poulose@xxxxxxx; > yuzenghui <yuzenghui@xxxxxxxxxx>; zhukeqian > <zhukeqian1@xxxxxxxxxx>; Jonathan Cameron > <jonathan.cameron@xxxxxxxxxx>; Linuxarm <linuxarm@xxxxxxxxxx> > Subject: Re: [RFC PATCH v2 3/8] KVM: arm64: Add some HW_DBM related > pgtable interfaces > > 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. > One of the other place we have mark_page_dirty() is in the stage2_unmap_walker(). And during the testing of this series, I have tried to remove that and found out that it actually causes memory corruption during VM migration. >From my old debug logs: [ 399.288076] stage2_unmap_walker+0x270/0x284 [ 399.288078] __kvm_pgtable_walk+0x1ec/0x2cc [ 399.288081] __kvm_pgtable_walk+0xec/0x2cc [ 399.288084] __kvm_pgtable_walk+0xec/0x2cc [ 399.288086] kvm_pgtable_walk+0xcc/0x160 [ 399.288088] kvm_pgtable_stage2_unmap+0x4c/0xbc [ 399.288091] stage2_apply_range+0xd0/0xec [ 399.288094] __unmap_stage2_range+0x2c/0x60 [ 399.288096] kvm_unmap_gfn_range+0x30/0x48 [ 399.288099] kvm_mmu_notifier_invalidate_range_start+0xe0/0x264 [ 399.288103] __mmu_notifier_invalidate_range_start+0xa4/0x23c [ 399.288106] change_protection+0x638/0x900 [ 399.288109] change_prot_numa+0x64/0xfc [ 399.288113] task_numa_work+0x2ac/0x450 [ 399.288117] task_work_run+0x78/0xd0 It looks like that the unmap path gets triggered from Numa page migration code path, so we may need it there. Thanks, Shameer
