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Fri, 07 Feb 2020 08:05:49 -0800 (PST) Received: from xz-x1 ([2607:9880:19c8:32::2]) by smtp.gmail.com with ESMTPSA id 69sm1462245qkg.133.2020.02.07.08.05.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 07 Feb 2020 08:05:48 -0800 (PST) Date: Fri, 7 Feb 2020 11:05:46 -0500 From: Peter Xu To: Sean Christopherson 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 =?utf-8?Q?Mathieu-Daud=C3=A9?= Subject: Re: [PATCH v5 18/19] KVM: Dynamically size memslot array based on number of used slots Message-ID: <20200207160546.GA707371@xz-x1> References: <20200121223157.15263-1-sean.j.christopherson@intel.com> <20200121223157.15263-19-sean.j.christopherson@intel.com> <20200206221208.GI700495@xz-x1> <20200207153829.GA2401@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200207153829.GA2401@linux.intel.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 07, 2020 at 07:38:29AM -0800, Sean Christopherson wrote: > 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. However... xz-x1:tmp $ cat a.c struct a { int s[]; }; int main(void) { } xz-x1:tmp $ make a cc a.c -o a a.c:2:9: error: flexible array member in a struct with no named members 2 | int s[]; | ^ make: *** [: a] Error 1 My gcc version is 9.2.1 20190827 (Red Hat 9.2.1-1) (GCC). > > > > }; > > > > > > 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. I made a silly mistake on reading "slots" as "old". Ignore my comment, sorry! And please take my R-b for this patch too: Reviewed-by: Peter Xu -- Peter Xu 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=-6.5 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 5D718C04EB5 for ; Fri, 7 Feb 2020 16:06:00 +0000 (UTC) Received: from mm01.cs.columbia.edu (mm01.cs.columbia.edu [128.59.11.253]) by mail.kernel.org (Postfix) with ESMTP id DB191214AF for ; Fri, 7 Feb 2020 16:05:59 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="egLsZbLP" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org DB191214AF Authentication-Results: mail.kernel.org; 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Fri, 07 Feb 2020 08:05:49 -0800 (PST) Received: from xz-x1 ([2607:9880:19c8:32::2]) by smtp.gmail.com with ESMTPSA id 69sm1462245qkg.133.2020.02.07.08.05.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 07 Feb 2020 08:05:48 -0800 (PST) Date: Fri, 7 Feb 2020 11:05:46 -0500 From: Peter Xu To: Sean Christopherson Subject: Re: [PATCH v5 18/19] KVM: Dynamically size memslot array based on number of used slots Message-ID: <20200207160546.GA707371@xz-x1> References: <20200121223157.15263-1-sean.j.christopherson@intel.com> <20200121223157.15263-19-sean.j.christopherson@intel.com> <20200206221208.GI700495@xz-x1> <20200207153829.GA2401@linux.intel.com> MIME-Version: 1.0 In-Reply-To: <20200207153829.GA2401@linux.intel.com> X-MC-Unique: 9zSr4ccKNWuqDKiA3tGzZw-1 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline Cc: Wanpeng Li , kvm@vger.kernel.org, David Hildenbrand , linux-mips@vger.kernel.org, Paul Mackerras , kvmarm@lists.cs.columbia.edu, Janosch Frank , Marc Zyngier , Joerg Roedel , Christian Borntraeger , kvm-ppc@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Jim Mattson , Cornelia Huck , linux-kernel@vger.kernel.org, Paolo Bonzini , Vitaly Kuznetsov , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= X-BeenThere: kvmarm@lists.cs.columbia.edu X-Mailman-Version: 2.1.14 Precedence: list List-Id: Where KVM/ARM decisions are made List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu On Fri, Feb 07, 2020 at 07:38:29AM -0800, Sean Christopherson wrote: > 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. However... xz-x1:tmp $ cat a.c struct a { int s[]; }; int main(void) { } xz-x1:tmp $ make a cc a.c -o a a.c:2:9: error: flexible array member in a struct with no named members 2 | int s[]; | ^ make: *** [: a] Error 1 My gcc version is 9.2.1 20190827 (Red Hat 9.2.1-1) (GCC). > > > > }; > > > > > > 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. I made a silly mistake on reading "slots" as "old". Ignore my comment, sorry! And please take my R-b for this patch too: Reviewed-by: Peter Xu -- Peter Xu _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm 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=-6.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 14189C04EB5 for ; Fri, 7 Feb 2020 16:06:08 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id DC4D0222D9 for ; 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Fri, 07 Feb 2020 08:05:50 -0800 (PST) X-Google-Smtp-Source: APXvYqyBU3fSbdWuwCNOt5og3uav2CnBhaKLUx7zFmWTkehMEXphwviUtToR7q9BtKv4R+OEmMKY/w== X-Received: by 2002:a37:c0c:: with SMTP id 12mr7542902qkm.425.1581091549699; Fri, 07 Feb 2020 08:05:49 -0800 (PST) Received: from xz-x1 ([2607:9880:19c8:32::2]) by smtp.gmail.com with ESMTPSA id 69sm1462245qkg.133.2020.02.07.08.05.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 07 Feb 2020 08:05:48 -0800 (PST) Date: Fri, 7 Feb 2020 11:05:46 -0500 From: Peter Xu To: Sean Christopherson Subject: Re: [PATCH v5 18/19] KVM: Dynamically size memslot array based on number of used slots Message-ID: <20200207160546.GA707371@xz-x1> References: <20200121223157.15263-1-sean.j.christopherson@intel.com> <20200121223157.15263-19-sean.j.christopherson@intel.com> <20200206221208.GI700495@xz-x1> <20200207153829.GA2401@linux.intel.com> MIME-Version: 1.0 In-Reply-To: <20200207153829.GA2401@linux.intel.com> X-MC-Unique: huNx2mHkM_OqMi_eC82K3g-1 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200207_080555_857804_3319620F X-CRM114-Status: GOOD ( 27.08 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Wanpeng Li , kvm@vger.kernel.org, David Hildenbrand , linux-mips@vger.kernel.org, Paul Mackerras , kvmarm@lists.cs.columbia.edu, Janosch Frank , Marc Zyngier , Joerg Roedel , Christian Borntraeger , Julien Thierry , Suzuki K Poulose , kvm-ppc@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Jim Mattson , Cornelia Huck , Christoffer Dall , linux-kernel@vger.kernel.org, James Morse , Paolo Bonzini , Vitaly Kuznetsov , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Fri, Feb 07, 2020 at 07:38:29AM -0800, Sean Christopherson wrote: > 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. However... xz-x1:tmp $ cat a.c struct a { int s[]; }; int main(void) { } xz-x1:tmp $ make a cc a.c -o a a.c:2:9: error: flexible array member in a struct with no named members 2 | int s[]; | ^ make: *** [: a] Error 1 My gcc version is 9.2.1 20190827 (Red Hat 9.2.1-1) (GCC). > > > > }; > > > > > > 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. I made a silly mistake on reading "slots" as "old". Ignore my comment, sorry! And please take my R-b for this patch too: Reviewed-by: Peter Xu -- Peter Xu _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Xu Date: Fri, 07 Feb 2020 16:05:46 +0000 Subject: Re: [PATCH v5 18/19] KVM: Dynamically size memslot array based on number of used slots Message-Id: <20200207160546.GA707371@xz-x1> List-Id: References: <20200121223157.15263-1-sean.j.christopherson@intel.com> <20200121223157.15263-19-sean.j.christopherson@intel.com> <20200206221208.GI700495@xz-x1> <20200207153829.GA2401@linux.intel.com> In-Reply-To: <20200207153829.GA2401@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Sean Christopherson 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 =?utf-8?Q?Mathieu-Daud=C3=A9?= On Fri, Feb 07, 2020 at 07:38:29AM -0800, Sean Christopherson wrote: > 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. However... xz-x1:tmp $ cat a.c struct a { int s[]; }; int main(void) { } xz-x1:tmp $ make a cc a.c -o a a.c:2:9: error: flexible array member in a struct with no named members 2 | int s[]; | ^ make: *** [: a] Error 1 My gcc version is 9.2.1 20190827 (Red Hat 9.2.1-1) (GCC). > > > > }; > > > > > > 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. I made a silly mistake on reading "slots" as "old". Ignore my comment, sorry! And please take my R-b for this patch too: Reviewed-by: Peter Xu -- Peter Xu