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 X-Spam-Level: X-Spam-Status: No, score=-11.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1AA2AC433E4 for ; Tue, 4 Aug 2020 21:18:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0EDAF22BF3 for ; Tue, 4 Aug 2020 21:18:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726568AbgHDVSt (ORCPT ); Tue, 4 Aug 2020 17:18:49 -0400 Received: from mga04.intel.com ([192.55.52.120]:25527 "EHLO mga04.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725863AbgHDVSs (ORCPT ); Tue, 4 Aug 2020 17:18:48 -0400 IronPort-SDR: 9LcTF7ZWoSA2TKv/tATLztz7IFmBoiUhQ6RzPtIIiYKPmLia+DN1ZDJzCLOzl2KslALLhtLn4P OArjT5pyYsJQ== X-IronPort-AV: E=McAfee;i="6000,8403,9703"; a="149856162" X-IronPort-AV: E=Sophos;i="5.75,435,1589266800"; d="scan'208,223";a="149856162" X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga002.fm.intel.com ([10.253.24.26]) by fmsmga104.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 04 Aug 2020 14:14:45 -0700 IronPort-SDR: s9YjZZBWjG5e6aJdDfVyX+cDOqgk/z2EpZKkEc28Xp7/3N+P0EUJTNbDS/VLAhp0CNPfpySdWY FvFoeZBQFuiA== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.75,435,1589266800"; d="scan'208,223";a="324799402" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.160]) by fmsmga002.fm.intel.com with ESMTP; 04 Aug 2020 14:14:45 -0700 Date: Tue, 4 Aug 2020 14:14:45 -0700 From: Sean Christopherson To: Ben Gardon Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, linux-kselftest@vger.kernel.org, Paolo Bonzini , Peter Xu , Peter Shier Subject: Re: [PATCH 1/1] kvm: mmu: zap pages when zapping only parent Message-ID: <20200804211444.GA31916@linux.intel.com> References: <20200727203324.2614917-1-bgardon@google.com> MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="cNdxnHkX5QqsyA0e" Content-Disposition: inline In-Reply-To: <20200727203324.2614917-1-bgardon@google.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kselftest-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kselftest@vger.kernel.org --cNdxnHkX5QqsyA0e Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Mon, Jul 27, 2020 at 01:33:24PM -0700, Ben Gardon wrote: > When the KVM MMU zaps a page, it will recursively zap the unsynced child > pages, but not the synced ones. This can create problems over time when > running many nested guests because it leaves unlinked pages which will not > be freed until the page quota is hit. With the default page quota of 20 > shadow pages per 1000 guest pages, this looks like a memory leak and can > degrade MMU performance. > > In a recent benchmark, substantial performance degradation was observed: > An L1 guest was booted with 64G memory. > 2G nested Windows guests were booted, 10 at a time for 20 > iterations. (200 total boots) > Windows was used in this benchmark because they touch all of their > memory on startup. > By the end of the benchmark, the nested guests were taking ~10% longer > to boot. With this patch there is no degradation in boot time. > Without this patch the benchmark ends with hundreds of thousands of > stale EPT02 pages cluttering up rmaps and the page hash map. As a > result, VM shutdown is also much slower: deleting memslot 0 was > observed to take over a minute. With this patch it takes just a > few miliseconds. > > If TDP is enabled, zap child shadow pages when zapping the only parent > shadow page. Comments on the mechanics below. For the approach itself, I wonder if we could/should go even further, i.e. be even more aggressive in reaping nested TDP shadow pages. For this to work, KVM is effectively relying on the write flooding detection in kvm_mmu_pte_write() to kick in, i.e. KVM needs the L1 VMM to overwrite the TDP tables that L1 was using for L2. In particular, L1 needs to write the upper level TDP entries in order for L0 to effeciently reclaim memory. For HyperV as L1, I believe that will happen sooner than later as HyperV maintains a pool of zeroed pages, i.e. L1 will quickly zero out the old TDP entries and trigger the zap. For KVM as L1, that may not hold true for all scenarios due to lazy zeroing of memory. If L1 is creating and destroying VMs (as in the benchmark), then it will work as expected. But if L1 creates and destroys a large L2 without reallocating all memory used for L2's TDP tables, the write flooding will never happen and L0 will keep the stale SPs even though L2 is dead. The above scenario may or may not be problematic in practice. I would assume any reasonably active L1 will quickly do something with the old TDP memory and trigger write flooding, but at the same time it's plausible that L1 could leave pages unused for a decent amount of time. One thought would be to track nested TDP PGDs and periodically purge PGDs that haven't been used in some arbitrary amount of time and/or an arbitrary threshold for the number of nested TDP PGDs is reached. That being said, either of those is probably overkill without more analysis on the below approach. > Tested by running the kvm-unit-tests suite on an Intel Haswell machine. > No regressions versus > commit c34b26b98cac ("KVM: MIPS: clean up redundant 'kvm_run' parameters"), > or warnings. > > Reviewed-by: Peter Shier > Signed-off-by: Ben Gardon > --- > arch/x86/kvm/mmu/mmu.c | 49 +++++++++++++++++++++++++++++++++++++----- > 1 file changed, 44 insertions(+), 5 deletions(-) > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c > index fa506aaaf0194..c550bc3831dcc 100644 > --- a/arch/x86/kvm/mmu/mmu.c > +++ b/arch/x86/kvm/mmu/mmu.c > @@ -2626,13 +2626,52 @@ static bool mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp, > return false; > } > > -static void kvm_mmu_page_unlink_children(struct kvm *kvm, > - struct kvm_mmu_page *sp) > +static int kvm_mmu_page_unlink_children(struct kvm *kvm, > + struct kvm_mmu_page *sp, > + struct list_head *invalid_list) > { > unsigned i; > + int zapped = 0; > + > + for (i = 0; i < PT64_ENT_PER_PAGE; ++i) { > + u64 *sptep = sp->spt + i; > + u64 spte = *sptep; > + struct kvm_mmu_page *child_sp; > + > + /* > + * Zap the page table entry, unlinking any potential child > + * page > + */ > + mmu_page_zap_pte(kvm, sp, sptep); > + > + /* If there is no child page for this spte, continue */ > + if (!is_shadow_present_pte(spte) || > + is_last_spte(spte, sp->role.level)) > + continue; > + > + /* > + * If TDP is enabled, then any shadow pages are part of either > + * the EPT01 or an EPT02. In either case, do not expect the > + * same pattern of page reuse seen in x86 PTs for > + * copy-on-write and similar techniques. In this case, it is > + * unlikely that a parentless shadow PT will be used again in > + * the near future. Zap it to keep the rmaps and page hash > + * maps from filling up with stale EPT02 pages. > + */ > + if (!tdp_enabled) > + continue; I haven't tested, but I believe this will have the unwanted side effect of blasting large swaths of EPT01 if recycling is triggered. Because the list of active SPs is FIFO (and never reordered), the first entries are almost always the root SP and then high level SPs. If make_mmu_pages_available() triggers kvm_mmu_zap_oldest_mmu_pages(), this is take out the high level SP and all its children. That may or may not be a problem in practice, but it's outside the scope of what this patch is trying to accomplish. TL;DR: what about further conditioning this on sp->role.guest_mode? > + > + child_sp = to_shadow_page(spte & PT64_BASE_ADDR_MASK); > + if (WARN_ON_ONCE(!child_sp)) This WARN is pointless, mmu_page_zap_pte() above will already have dereferenced the child shadow page, i.e. any null pointer will have exploded. > + continue; > + > + /* Zap the page if it has no remaining parent pages */ > + if (!child_sp->parent_ptes.val) IMO it's easier to read if these checks are collapsed, e.g.: if (!tdp_enabled || !child_sp->role.guest_mode || child_sp->parent_ptes.val) continue; zapped += kvm_mmu_prepare_zap_page(kvm, child_sp, invalid_list); Alternatively, what about moving this logic into mmu_page_zap_pte()? That can be done with a little massaging of FNAME(invlpg) and would avoid what is effectively redundant checks on is_shadow_present_pte() and is_last_spte(). Patches attached and somewhat tested. > + zapped += kvm_mmu_prepare_zap_page(kvm, child_sp, > + invalid_list); > + } > > - for (i = 0; i < PT64_ENT_PER_PAGE; ++i) > - mmu_page_zap_pte(kvm, sp, sp->spt + i); > + return zapped; > } > > static void kvm_mmu_unlink_parents(struct kvm *kvm, struct kvm_mmu_page *sp) > @@ -2678,7 +2717,7 @@ static bool __kvm_mmu_prepare_zap_page(struct kvm *kvm, > trace_kvm_mmu_prepare_zap_page(sp); > ++kvm->stat.mmu_shadow_zapped; > *nr_zapped = mmu_zap_unsync_children(kvm, sp, invalid_list); > - kvm_mmu_page_unlink_children(kvm, sp); > + *nr_zapped += kvm_mmu_page_unlink_children(kvm, sp, invalid_list); > kvm_mmu_unlink_parents(kvm, sp); > > /* Zapping children means active_mmu_pages has become unstable. */ > -- > 2.28.0.rc0.142.g3c755180ce-goog > --cNdxnHkX5QqsyA0e Content-Type: text/x-diff; charset=us-ascii Content-Disposition: attachment; filename="0001-KVM-x86-mmu-Move-flush-logic-from-mmu_page_zap_pte-t.patch" >From b1964178d9b6abadd33531a5dd7788368f89bc40 Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Tue, 4 Aug 2020 09:14:21 -0700 Subject: [PATCH 1/2] KVM: x86/mmu: Move flush logic from mmu_page_zap_pte() to FNAME(invlpg) Move the logic that controls whether or not FNAME(invlpg) needs to flush fully into FNAME(invlpg) so that mmu_page_zap_pte() doesn't return a value. This allows a future patch to redefine the return semantics for mmu_page_zap_pte() so that it can recursively zap orphaned child shadow pages for nested TDP MMUs. No functional change intended. Signed-off-by: Sean Christopherson --- arch/x86/kvm/mmu/mmu.c | 10 +++------- arch/x86/kvm/mmu/paging_tmpl.h | 7 +++++-- 2 files changed, 8 insertions(+), 9 deletions(-) diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 862bf418214e2..bdc10a53cd6ad 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -2614,7 +2614,7 @@ static void validate_direct_spte(struct kvm_vcpu *vcpu, u64 *sptep, } } -static bool mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp, +static void mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp, u64 *spte) { u64 pte; @@ -2630,13 +2630,9 @@ static bool mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp, child = to_shadow_page(pte & PT64_BASE_ADDR_MASK); drop_parent_pte(child, spte); } - return true; - } - - if (is_mmio_spte(pte)) + } else if (is_mmio_spte(pte)) { mmu_spte_clear_no_track(spte); - - return false; + } } static void kvm_mmu_page_unlink_children(struct kvm *kvm, diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h index 0172a949f6a75..1fe7edc5732da 100644 --- a/arch/x86/kvm/mmu/paging_tmpl.h +++ b/arch/x86/kvm/mmu/paging_tmpl.h @@ -895,6 +895,7 @@ static void FNAME(invlpg)(struct kvm_vcpu *vcpu, gva_t gva, hpa_t root_hpa) { struct kvm_shadow_walk_iterator iterator; struct kvm_mmu_page *sp; + u64 old_spte; int level; u64 *sptep; @@ -917,7 +918,8 @@ static void FNAME(invlpg)(struct kvm_vcpu *vcpu, gva_t gva, hpa_t root_hpa) sptep = iterator.sptep; sp = sptep_to_sp(sptep); - if (is_last_spte(*sptep, level)) { + old_spte = *sptep; + if (is_last_spte(old_spte, level)) { pt_element_t gpte; gpa_t pte_gpa; @@ -927,7 +929,8 @@ static void FNAME(invlpg)(struct kvm_vcpu *vcpu, gva_t gva, hpa_t root_hpa) pte_gpa = FNAME(get_level1_sp_gpa)(sp); pte_gpa += (sptep - sp->spt) * sizeof(pt_element_t); - if (mmu_page_zap_pte(vcpu->kvm, sp, sptep)) + mmu_page_zap_pte(vcpu->kvm, sp, sptep); + if (is_shadow_present_pte(old_spte)) kvm_flush_remote_tlbs_with_address(vcpu->kvm, sp->gfn, KVM_PAGES_PER_HPAGE(sp->role.level)); -- 2.28.0 --cNdxnHkX5QqsyA0e Content-Type: text/x-diff; charset=us-ascii Content-Disposition: attachment; filename="0002-KVM-x86-MMU-Recursively-zap-nested-TDP-SPs-when-zapp.patch" >From 848917abc9c928df254d77a6fa5a5d874461975e Mon Sep 17 00:00:00 2001 From: Ben Gardon Date: Tue, 4 Aug 2020 10:02:17 -0700 Subject: [PATCH 2/2] KVM: x86/MMU: Recursively zap nested TDP SPs when zapping last/only parent Recursively zap all to-be-orphaned children, unsynced or otherwise, when zapping a shadow page for a nested TDP MMU. KVM currently only zaps the unsynced child pages, but not the synced ones. This can create problems over time when running many nested guests because it leaves unlinked pages which will not be freed until the page quota is hit. With the default page quota of 20 shadow pages per 1000 guest pages, this looks like a memory leak and can degrade MMU performance. In a recent benchmark, substantial performance degradation was observed: An L1 guest was booted with 64G memory. 2G nested Windows guests were booted, 10 at a time for 20 iterations. (200 total boots) Windows was used in this benchmark because they touch all of their memory on startup. By the end of the benchmark, the nested guests were taking ~10% longer to boot. With this patch there is no degradation in boot time. Without this patch the benchmark ends with hundreds of thousands of stale EPT02 pages cluttering up rmaps and the page hash map. As a result, VM shutdown is also much slower: deleting memslot 0 was observed to take over a minute. With this patch it takes just a few miliseconds. Cc: Peter Shier Signed-off-by: Ben Gardon Co-developed-by: Sean Christopherson Signed-off-by: Sean Christopherson --- arch/x86/kvm/mmu/mmu.c | 30 +++++++++++++++++++++++------- arch/x86/kvm/mmu/paging_tmpl.h | 2 +- 2 files changed, 24 insertions(+), 8 deletions(-) diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index bdc10a53cd6ad..11d3c6ec608ef 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -2614,8 +2614,9 @@ static void validate_direct_spte(struct kvm_vcpu *vcpu, u64 *sptep, } } -static void mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp, - u64 *spte) +/* Returns the number of zapped non-leaf child shadow pages. */ +static int mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp, + u64 *spte, struct list_head *invalid_list) { u64 pte; struct kvm_mmu_page *child; @@ -2629,19 +2630,34 @@ static void mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp, } else { child = to_shadow_page(pte & PT64_BASE_ADDR_MASK); drop_parent_pte(child, spte); + + /* + * Recursively zap nested TDP SPs, parentless SPs are + * unlikely to be used again in the near future. This + * avoids retaining a large number of stale nested SPs. + */ + if (tdp_enabled && invalid_list && + child->role.guest_mode && !child->parent_ptes.val) + return kvm_mmu_prepare_zap_page(kvm, child, + invalid_list); } } else if (is_mmio_spte(pte)) { mmu_spte_clear_no_track(spte); } + return 0; } -static void kvm_mmu_page_unlink_children(struct kvm *kvm, - struct kvm_mmu_page *sp) +static int kvm_mmu_page_unlink_children(struct kvm *kvm, + struct kvm_mmu_page *sp, + struct list_head *invalid_list) { + int zapped = 0; unsigned i; for (i = 0; i < PT64_ENT_PER_PAGE; ++i) - mmu_page_zap_pte(kvm, sp, sp->spt + i); + zapped += mmu_page_zap_pte(kvm, sp, sp->spt + i, invalid_list); + + return zapped; } static void kvm_mmu_unlink_parents(struct kvm *kvm, struct kvm_mmu_page *sp) @@ -2687,7 +2703,7 @@ static bool __kvm_mmu_prepare_zap_page(struct kvm *kvm, trace_kvm_mmu_prepare_zap_page(sp); ++kvm->stat.mmu_shadow_zapped; *nr_zapped = mmu_zap_unsync_children(kvm, sp, invalid_list); - kvm_mmu_page_unlink_children(kvm, sp); + *nr_zapped += kvm_mmu_page_unlink_children(kvm, sp, invalid_list); kvm_mmu_unlink_parents(kvm, sp); /* Zapping children means active_mmu_pages has become unstable. */ @@ -5395,7 +5411,7 @@ static void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa, u32 base_role = vcpu->arch.mmu->mmu_role.base.word; entry = *spte; - mmu_page_zap_pte(vcpu->kvm, sp, spte); + mmu_page_zap_pte(vcpu->kvm, sp, spte, NULL); if (gentry && !((sp->role.word ^ base_role) & ~role_ign.word) && rmap_can_add(vcpu)) diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h index 1fe7edc5732da..58ae852a9615b 100644 --- a/arch/x86/kvm/mmu/paging_tmpl.h +++ b/arch/x86/kvm/mmu/paging_tmpl.h @@ -929,7 +929,7 @@ static void FNAME(invlpg)(struct kvm_vcpu *vcpu, gva_t gva, hpa_t root_hpa) pte_gpa = FNAME(get_level1_sp_gpa)(sp); pte_gpa += (sptep - sp->spt) * sizeof(pt_element_t); - mmu_page_zap_pte(vcpu->kvm, sp, sptep); + mmu_page_zap_pte(vcpu->kvm, sp, sptep, NULL); if (is_shadow_present_pte(old_spte)) kvm_flush_remote_tlbs_with_address(vcpu->kvm, sp->gfn, KVM_PAGES_PER_HPAGE(sp->role.level)); -- 2.28.0 --cNdxnHkX5QqsyA0e--