From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 55462E7E648 for ; Tue, 26 Sep 2023 15:20:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233309AbjIZPUR (ORCPT ); Tue, 26 Sep 2023 11:20:17 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39462 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229732AbjIZPUP (ORCPT ); Tue, 26 Sep 2023 11:20:15 -0400 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DE31410A for ; Tue, 26 Sep 2023 08:20:08 -0700 (PDT) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 117E9C433C8; Tue, 26 Sep 2023 15:20:05 +0000 (UTC) Date: Tue, 26 Sep 2023 16:20:03 +0100 From: Catalin Marinas To: Shameerali Kolothum Thodi Cc: Oliver Upton , "kvmarm@lists.linux.dev" , "kvm@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "maz@kernel.org" , "will@kernel.org" , "james.morse@arm.com" , "suzuki.poulose@arm.com" , yuzenghui , zhukeqian , Jonathan Cameron , Linuxarm Subject: Re: [RFC PATCH v2 3/8] KVM: arm64: Add some HW_DBM related pgtable interfaces Message-ID: References: <20230825093528.1637-1-shameerali.kolothum.thodi@huawei.com> <20230825093528.1637-4-shameerali.kolothum.thodi@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Mon, Sep 25, 2023 at 08:04:39AM +0000, Shameerali Kolothum Thodi wrote: > From: Oliver Upton [mailto:oliver.upton@linux.dev] > > 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