[PATCH v5 20/21] KVM: Allow for different capacities in kvm_mmu_memory_cache structs

[David Matlack]

On Mon, May 16, 2022 at 7:49 AM Sean Christopherson <seanjc at google.com> wrote:
>
> On Fri, May 13, 2022, David Matlack wrote:
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index 53ae2c0640bc..2f2ef6b60ff4 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -764,7 +764,10 @@ 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 = {
> > +             .capacity = KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE,
> > +             .gfp_zero = __GFP_ZERO,
>
> I dislike requiring all users to specificy the capacity.  It largely defeats the
> purpose of KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE, and bleeds details into code that
> really doesn't care all that much about the details.
>
> Rather than force the capacity to be set before topup, what about adding a custom
> capacity topup helper?  That allows keeping a default capacity, better documents
> the caches that are special, and provides an opportunity to sanity check that the
> capacity isn't incorrectly changed by the user.

Even simpler: If mc->capacity is 0 in topup, set it to
KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE.

This is what I had before when I was laying out the storage for
objects in a separate array. It was risky then because it was too easy
for someone to accidentally corrupt memory (call topup with
capacity==0 but without allocating the objects array). Now that topup
takes care of allocation automatically, that risk is gone.

>
> And then I believe this code becomes:
>
>         struct kvm_mmu_memory_cache cache = { .gfp_zero = __GFP_ZERO };
>
> E.g. (completely untested)
>
> 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;
>
>         if (likely(mc->capacity)) {
>                 if (WARN_ON_ONCE(mc->capacity != capacity || !mc->objects))
>                         return -EIO;
>         } else {
>                 mc->objects = kvmalloc_array(sizeof(void *), capacity, gfp);
>                 if (!mc->objects)
>                         return -ENOMEM;
>
>                 mc->capacity = capacity;
>         }
>
>         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;
>         }
>         return 0;
> }
>
> int kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int min)
> {
>         const int capacity = KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE;
>
>         return __kvm_mmu_topup_memory_cache(mc, capacity, min);
> }
>
> int kvm_mmu_topup_custom_memory_cache(struct kvm_mmu_memory_cache *mc,
>                                       int capacity)
> {
>         return __kvm_mmu_topup_memory_cache(mc, capacity, capacity);
> }
>