On Fri, Mar 4, 2022 at 1:59 PM David Matlack <dmatlack@xxxxxxxxxx> wrote: > > On Thu, Feb 24, 2022 at 11:20 AM David Matlack <dmatlack@xxxxxxxxxx> wrote: > > > > On Thu, Feb 24, 2022 at 3:29 AM Marc Zyngier <maz@xxxxxxxxxx> wrote: > > > > > > On Thu, 03 Feb 2022 01:00:47 +0000, > > > David Matlack <dmatlack@xxxxxxxxxx> wrote: > > > > > > [...] > > > > > > > > > /* Cache some mmu pages needed inside spinlock regions */ > > > > - struct kvm_mmu_memory_cache mmu_page_cache; > > > > + DEFINE_KVM_MMU_MEMORY_CACHE(mmu_page_cache); > > > > > > I must say I'm really not a fan of the anonymous structure trick. I > > > can see why you are doing it that way, but it feels pretty brittle. > > > > Yeah I don't love it. It's really optimizing for minimizing the patch diff. > > > > The alternative I considered was to dynamically allocate the > > kvm_mmu_memory_cache structs. This would get rid of the anonymous > > struct and the objects array, and also eliminate the rather gross > > capacity hack in kvm_mmu_topup_memory_cache(). > > > > The downsides of this approach is more code and more failure paths if > > the allocation fails. > > I tried changing all kvm_mmu_memory_cache structs to be dynamically > allocated, but it created a lot of complexity to the setup/teardown > code paths in x86, arm64, mips, and riscv (the arches that use the > caches). I don't think this route is worth it, especially since these > structs don't *need* to be dynamically allocated. > > When you said the anonymous struct feels brittle, what did you have in > mind specifically? > > > > > > > > > > > > > > /* Target CPU and feature flags */ > > > > int target; > > > > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c > > > > index bc2aba953299..9c853c529b49 100644 > > > > --- a/arch/arm64/kvm/mmu.c > > > > +++ b/arch/arm64/kvm/mmu.c > > > > @@ -765,7 +765,8 @@ int kvm_phys_addr_ioremap(struct kvm *kvm, phys_addr_t guest_ipa, > > > > { > > > > phys_addr_t addr; > > > > int ret = 0; > > > > - struct kvm_mmu_memory_cache cache = { 0, __GFP_ZERO, NULL, }; > > > > + DEFINE_KVM_MMU_MEMORY_CACHE(cache) page_cache = {}; > > > > + struct kvm_mmu_memory_cache *cache = &page_cache.cache; > > > > struct kvm_pgtable *pgt = kvm->arch.mmu.pgt; > > > > enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_DEVICE | > > > > KVM_PGTABLE_PROT_R | > > > > @@ -774,18 +775,17 @@ int kvm_phys_addr_ioremap(struct kvm *kvm, phys_addr_t guest_ipa, > > > > if (is_protected_kvm_enabled()) > > > > return -EPERM; > > > > > > > > + cache->gfp_zero = __GFP_ZERO; > > > > > > nit: consider this instead, which preserves the existing flow: > > > > Will do. > > > > > > > > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c > > > index 26d6c53be083..86a7ebd03a44 100644 > > > --- a/arch/arm64/kvm/mmu.c > > > +++ b/arch/arm64/kvm/mmu.c > > > @@ -764,7 +764,9 @@ int kvm_phys_addr_ioremap(struct kvm *kvm, phys_addr_t guest_ipa, > > > { > > > phys_addr_t addr; > > > int ret = 0; > > > - DEFINE_KVM_MMU_MEMORY_CACHE(cache) page_cache = {}; > > > + DEFINE_KVM_MMU_MEMORY_CACHE(cache) page_cache = { > > > + .cache = { .gfp_zero = __GFP_ZERO}, > > > + }; > > > struct kvm_mmu_memory_cache *cache = &page_cache.cache; > > > struct kvm_pgtable *pgt = kvm->arch.mmu.pgt; > > > enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_DEVICE | > > > @@ -774,7 +776,6 @@ int kvm_phys_addr_ioremap(struct kvm *kvm, phys_addr_t guest_ipa, > > > if (is_protected_kvm_enabled()) > > > return -EPERM; > > > > > > - cache->gfp_zero = __GFP_ZERO; > > > size += offset_in_page(guest_ipa); > > > guest_ipa &= PAGE_MASK; > > > > > > but whole "declare the outer structure and just use the inner one" > > > hack is... huh... :-/ > > > > Yeah it's not great. Unfortunately (or maybe fortunately?) anonymous > > structs cannot be defined in functions. So naming the outer struct is > > necessary even though we only need to use the inner one. > > I see two alternatives to make this cleaner: > > 1. Dynamically allocate just this cache. The caches defined in > vcpu_arch will continue to use DEFINE_KVM_MMU_MEMORY_CACHE(). This > would get rid of the outer struct but require an extra memory > allocation. > 2. Move this cache to struct kvm_arch using > DEFINE_KVM_MMU_MEMORY_CACHE(). Then we don't need to stack allocate it > or dynamically allocate it. > > Do either of these approaches appeal to you more than the current one? (There's obvious performance and memory overhead trade-offs with these different approaches, but I don't know enough about arm64 KVM to assess which option might be best.) > > > > > > > > > This hunk also conflicts with what currently sits in -next. Not a big > > > deal, but just so you know. > > > > Ack. > > > > > > > > > diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h > > > > index dceac12c1ce5..9575fb8d333f 100644 > > > > --- a/include/linux/kvm_types.h > > > > +++ b/include/linux/kvm_types.h > > > > @@ -78,14 +78,34 @@ struct gfn_to_pfn_cache { > > > > * MMU flows is problematic, as is triggering reclaim, I/O, etc... while > > > > * holding MMU locks. Note, these caches act more like prefetch buffers than > > > > * classical caches, i.e. objects are not returned to the cache on being freed. > > > > + * > > > > + * The storage for the cache objects is laid out after the struct to allow > > > > + * different declarations to choose different capacities. If the capacity field > > > > + * is 0, the capacity is assumed to be KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE. > > > > */ > > > > struct kvm_mmu_memory_cache { > > > > int nobjs; > > > > + int capacity; > > > > gfp_t gfp_zero; > > > > struct kmem_cache *kmem_cache; > > > > - void *objects[KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE]; > > > > + void *objects[0]; > > > > > > The VLA police is going to track you down ([0] vs []). > > > > Thanks! > > > > > > > > > > M. > > > > > > -- > > > Without deviation from the norm, progress is not possible.