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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: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@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