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=-8.2 required=3.0 tests=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 62052C04EB5 for ; Fri, 7 Feb 2020 15:38:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3AEC6214AF for ; Fri, 7 Feb 2020 15:38:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727390AbgBGPib (ORCPT ); Fri, 7 Feb 2020 10:38:31 -0500 Received: from mga17.intel.com ([192.55.52.151]:52915 "EHLO mga17.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726982AbgBGPib (ORCPT ); Fri, 7 Feb 2020 10:38:31 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga004.fm.intel.com ([10.253.24.48]) by fmsmga107.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 07 Feb 2020 07:38:30 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.70,413,1574150400"; d="scan'208";a="255445625" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.202]) by fmsmga004.fm.intel.com with ESMTP; 07 Feb 2020 07:38:29 -0800 Date: Fri, 7 Feb 2020 07:38:29 -0800 From: Sean Christopherson To: Peter Xu Cc: Paolo Bonzini , Paul Mackerras , Christian Borntraeger , Janosch Frank , David Hildenbrand , Cornelia Huck , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Marc Zyngier , James Morse , Julien Thierry , Suzuki K Poulose , linux-mips@vger.kernel.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org, Christoffer Dall , Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= Subject: Re: [PATCH v5 18/19] KVM: Dynamically size memslot array based on number of used slots Message-ID: <20200207153829.GA2401@linux.intel.com> References: <20200121223157.15263-1-sean.j.christopherson@intel.com> <20200121223157.15263-19-sean.j.christopherson@intel.com> <20200206221208.GI700495@xz-x1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200206221208.GI700495@xz-x1> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 06, 2020 at 05:12:08PM -0500, Peter Xu wrote: > On Tue, Jan 21, 2020 at 02:31:56PM -0800, Sean Christopherson wrote: > > Now that the memslot logic doesn't assume memslots are always non-NULL, > > dynamically size the array of memslots instead of unconditionally > > allocating memory for the maximum number of memslots. > > > > Note, because a to-be-deleted memslot must first be invalidated, the > > array size cannot be immediately reduced when deleting a memslot. > > However, consecutive deletions will realize the memory savings, i.e. > > a second deletion will trim the entry. > > > > Tested-by: Christoffer Dall > > Tested-by: Marc Zyngier > > Signed-off-by: Sean Christopherson > > --- > > include/linux/kvm_host.h | 2 +- > > virt/kvm/kvm_main.c | 31 ++++++++++++++++++++++++++++--- > > 2 files changed, 29 insertions(+), 4 deletions(-) > > > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > > index 60ddfdb69378..8bb6fb127387 100644 > > --- a/include/linux/kvm_host.h > > +++ b/include/linux/kvm_host.h > > @@ -431,11 +431,11 @@ static inline int kvm_arch_vcpu_memslots_id(struct kvm_vcpu *vcpu) > > */ > > struct kvm_memslots { > > u64 generation; > > - struct kvm_memory_slot memslots[KVM_MEM_SLOTS_NUM]; > > /* The mapping table from slot id to the index in memslots[]. */ > > short id_to_index[KVM_MEM_SLOTS_NUM]; > > atomic_t lru_slot; > > int used_slots; > > + struct kvm_memory_slot memslots[]; > > This patch is tested so I believe this works, however normally I need > to do similar thing with [0] otherwise gcc might complaint. Is there > any trick behind to make this work? Or is that because of different > gcc versions? array[] and array[0] have the same net affect, but array[] is given special treatment by gcc to provide extra sanity checks, e.g. requires the field to be the end of the struct. Last I checked, gcc also doesn't allow array[] in unions. There are probably other restrictions. But, it's precisely because of those restrictions that using array[] is preferred, as it provides extra protections, e.g. if someone moved memslots to the top of the struct it would fail to compile. > > }; > > > > struct kvm { > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > > index 9b614cf2ca20..ed392ce64e59 100644 > > --- a/virt/kvm/kvm_main.c > > +++ b/virt/kvm/kvm_main.c > > @@ -565,7 +565,7 @@ static struct kvm_memslots *kvm_alloc_memslots(void) > > return NULL; > > > > for (i = 0; i < KVM_MEM_SLOTS_NUM; i++) > > - slots->id_to_index[i] = slots->memslots[i].id = -1; > > + slots->id_to_index[i] = -1; > > > > return slots; > > } > > @@ -1077,6 +1077,32 @@ static struct kvm_memslots *install_new_memslots(struct kvm *kvm, > > return old_memslots; > > } > > > > +/* > > + * Note, at a minimum, the current number of used slots must be allocated, even > > + * when deleting a memslot, as we need a complete duplicate of the memslots for > > + * use when invalidating a memslot prior to deleting/moving the memslot. > > + */ > > +static struct kvm_memslots *kvm_dup_memslots(struct kvm_memslots *old, > > + enum kvm_mr_change change) > > +{ > > + struct kvm_memslots *slots; > > + size_t old_size, new_size; > > + > > + old_size = sizeof(struct kvm_memslots) + > > + (sizeof(struct kvm_memory_slot) * old->used_slots); > > + > > + if (change == KVM_MR_CREATE) > > + new_size = old_size + sizeof(struct kvm_memory_slot); > > + else > > + new_size = old_size; > > + > > + slots = kvzalloc(new_size, GFP_KERNEL_ACCOUNT); > > + if (likely(slots)) > > + memcpy(slots, old, old_size); > > (Maybe directly copy into it?) I don't follow, are you saying do "*slots = *old"? @new_size and @old_size are not guaranteed to be the same. More specifically, slots->memslots and old->slots are now flexible arrays with potentially different sizes. Doing "*slots = *old" would only copy the standard members, a memcpy() would still be needed for @memlots. A more effecient implementation would be: slots = kvalloc(new_size, GFP_KERNEL_ACCOUNT); if (likely(slots)) { memcpy(slots, old, old_size); if (change == KVM_MR_CREATE) memset((void *)slots + old_size, 0, new_size - old_size); } to avoid unnecessarily zeroing out the entire thing. I opted for the simpler implementation as this is not performance critical code, for most cases @slots won't be all that large, and I wanted to be absolutely sure any mixup would hit zeroed memory and not uninitialized memory. > > > + > > + return slots; > > +} > > + > > static int kvm_set_memslot(struct kvm *kvm, > > const struct kvm_userspace_memory_region *mem, > > struct kvm_memory_slot *old, > > @@ -1087,10 +1113,9 @@ static int kvm_set_memslot(struct kvm *kvm, > > struct kvm_memslots *slots; > > int r; > > > > - slots = kvzalloc(sizeof(struct kvm_memslots), GFP_KERNEL_ACCOUNT); > > + slots = kvm_dup_memslots(__kvm_memslots(kvm, as_id), change); > > if (!slots) > > return -ENOMEM; > > - memcpy(slots, __kvm_memslots(kvm, as_id), sizeof(struct kvm_memslots)); > > > > if (change == KVM_MR_DELETE || change == KVM_MR_MOVE) { > > /* > > -- > > 2.24.1 > > > > -- > Peter Xu >