Re: [PATCH v6 21/22] KVM: Allow for different capacities in kvm_mmu_memory_cache structs

[Mingwei Zhang <mizhang@google.com>]

On Mon, May 16, 2022 at 4:24 PM David Matlack <dmatlack@google.com> wrote:
>
> Allow the capacity of the kvm_mmu_memory_cache struct to be chosen at
> declaration time rather than being fixed for all declarations. This will
> be used in a follow-up commit to declare an cache in x86 with a capacity
> of 512+ objects without having to increase the capacity of all caches in
> KVM.
>
> This change requires each cache now specify its capacity at runtime,
> since the cache struct itself no longer has a fixed capacity known at
> compile time. To protect against someone accidentally defining a
> kvm_mmu_memory_cache struct directly (without the extra storage), this
> commit includes a WARN_ON() in kvm_mmu_topup_memory_cache().
>
> In order to support different capacities, this commit changes the
> objects pointer array to be dynamically allocated the first time the
> cache is topped-up.
>
> While here, opportunistically clean up the stack-allocated
> kvm_mmu_memory_cache structs in riscv and arm64 to use designated
> initializers.
>
> No functional change intended.
>
> Reviewed-by: Marc Zyngier <maz@kernel.org>
> Signed-off-by: David Matlack <dmatlack@google.com>
> ---
>  arch/arm64/kvm/mmu.c      |  2 +-
>  arch/riscv/kvm/mmu.c      |  5 +----
>  include/linux/kvm_types.h |  6 +++++-
>  virt/kvm/kvm_main.c       | 33 ++++++++++++++++++++++++++++++---
>  4 files changed, 37 insertions(+), 9 deletions(-)
>
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 53ae2c0640bc..f443ed845f85 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -764,7 +764,7 @@ 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, };
> +       struct kvm_mmu_memory_cache cache = { .gfp_zero = __GFP_ZERO };
>         struct kvm_pgtable *pgt = kvm->arch.mmu.pgt;
>         enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_DEVICE |
>                                      KVM_PGTABLE_PROT_R |
> diff --git a/arch/riscv/kvm/mmu.c b/arch/riscv/kvm/mmu.c
> index f80a34fbf102..4d95ebe4114f 100644
> --- a/arch/riscv/kvm/mmu.c
> +++ b/arch/riscv/kvm/mmu.c
> @@ -347,10 +347,7 @@ static int stage2_ioremap(struct kvm *kvm, gpa_t gpa, phys_addr_t hpa,
>         int ret = 0;
>         unsigned long pfn;
>         phys_addr_t addr, end;
> -       struct kvm_mmu_memory_cache pcache;
> -
> -       memset(&pcache, 0, sizeof(pcache));
> -       pcache.gfp_zero = __GFP_ZERO;
> +       struct kvm_mmu_memory_cache pcache = { .gfp_zero = __GFP_ZERO };
>
>         end = (gpa + size + PAGE_SIZE - 1) & PAGE_MASK;
>         pfn = __phys_to_pfn(hpa);
> diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
> index ac1ebb37a0ff..68529884eaf8 100644
> --- a/include/linux/kvm_types.h
> +++ b/include/linux/kvm_types.h
> @@ -83,12 +83,16 @@ 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 @capacity field and @objects array are lazily initialized when the cache
> + * is topped up (__kvm_mmu_topup_memory_cache()).
>   */
>  struct kvm_mmu_memory_cache {
>         int nobjs;
>         gfp_t gfp_zero;
>         struct kmem_cache *kmem_cache;
> -       void *objects[KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE];
> +       int capacity;
> +       void **objects;
>  };
>  #endif
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index e089db822c12..5e2e75014256 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -369,14 +369,31 @@ static inline void *mmu_memory_cache_alloc_obj(struct kvm_mmu_memory_cache *mc,
>                 return (void *)__get_free_page(gfp_flags);
>  }
>
> -int kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int min)
> +static int __kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int capacity, int min)
>  {
> +       gfp_t gfp = GFP_KERNEL_ACCOUNT;
>         void *obj;
>
>         if (mc->nobjs >= min)
>                 return 0;
> -       while (mc->nobjs < ARRAY_SIZE(mc->objects)) {
> -               obj = mmu_memory_cache_alloc_obj(mc, GFP_KERNEL_ACCOUNT);
> +
> +       if (unlikely(!mc->objects)) {
> +               if (WARN_ON_ONCE(!capacity))
> +                       return -EIO;
> +
> +               mc->objects = kvmalloc_array(sizeof(void *), capacity, gfp);
> +               if (!mc->objects)
> +                       return -ENOMEM;
> +
> +               mc->capacity = capacity;

Do we want to ensure the minimum value of the capacity? I think
otherwise, we may more likely start using memory from GFP_ATOMIC if
the capacity is less than, say 5? But the minimum value seems related
to each cache type.

> +       }
> +
> +       /* It is illegal to request a different capacity across topups. */
> +       if (WARN_ON_ONCE(mc->capacity != capacity))
> +               return -EIO;
> +
> +       while (mc->nobjs < mc->capacity) {
> +               obj = mmu_memory_cache_alloc_obj(mc, gfp);
>                 if (!obj)
>                         return mc->nobjs >= min ? 0 : -ENOMEM;
>                 mc->objects[mc->nobjs++] = obj;
> @@ -384,6 +401,11 @@ int kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int min)
>         return 0;
>  }
>
> +int kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int min)
> +{
> +       return __kvm_mmu_topup_memory_cache(mc, KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE, min);
> +}
> +
>  int kvm_mmu_memory_cache_nr_free_objects(struct kvm_mmu_memory_cache *mc)
>  {
>         return mc->nobjs;
> @@ -397,6 +419,11 @@ void kvm_mmu_free_memory_cache(struct kvm_mmu_memory_cache *mc)
>                 else
>                         free_page((unsigned long)mc->objects[--mc->nobjs]);
>         }
> +
> +       kvfree(mc->objects);
> +
> +       mc->objects = NULL;
> +       mc->capacity = 0;
>  }
>
>  void *kvm_mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc)
> --
> 2.36.0.550.gb090851708-goog
>