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

David Matlack
2022-05-16T23:24:17Z

On Sun, May 15, 2022 at 8:32 PM Anup Patel <anup@brainfault.org> wrote:
>
> On Sat, May 14, 2022 at 1:59 AM 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.
> >
> > An alternative would be to lay out the objects array after the
> > kvm_mmu_memory_cache struct, which can be done at compile time. But that
> > change, unfortunately, adds some grottiness to arm64 and riscv, which
> > uses a function-local (i.e. stack-allocated) kvm_mmu_memory_cache
> > struct. Since C does not allow anonymous structs in functions, the new
> > wrapper struct that contains kvm_mmu_memory_cache and the objects
> > pointer array, must be named, which means dealing with an outer and
> > inner struct. The outer struct can't be dropped since then there would
> > be no guarantee the kvm_mmu_memory_cache struct and objects array would
> > be laid out consecutively on the stack.
> >
> > No functional change intended.
> >
> > Signed-off-by: David Matlack <dmatlack@google.com>
>
> Overall, this looks good to me. I wanted to try this patch with KVM RISC-V
> but there are conflicts in include/linux/kvm_types.h and virt/kvm/kvm_main.c

Hi Anup,

I just sent another version rebased on top of the latest kvm/queue.
Let me know if you are still seeing merge conflicts.

>
> Please take a look.
>
> Regards,
> Anup
>
> > ---
> > arch/arm64/kvm/arm.c | 1 +
> > arch/arm64/kvm/mmu.c | 5 ++++-
> > arch/mips/kvm/mips.c | 2 ++
> > arch/riscv/kvm/mmu.c | 8 ++++----
> > arch/riscv/kvm/vcpu.c | 1 +
> > arch/x86/kvm/mmu/mmu.c | 9 +++++++++
> > include/linux/kvm_types.h | 9 +++++++--
> > virt/kvm/kvm_main.c | 20 ++++++++++++++++++--
> > 8 files changed, 46 insertions(+), 9 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index 7fceb855fa71..aa1e0c1659d4 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -320,6 +320,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
> > vcpu->arch.target = -1;
> > bitmap_zero(vcpu->arch.features, KVM_VCPU_MAX_FEATURES);
> >
> > + vcpu->arch.mmu_page_cache.capacity = KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE;
> > vcpu->arch.mmu_page_cache.gfp_zero = __GFP_ZERO;
> >
> > /* Set up the timer */
> > 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,
> > + };
> > 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/mips/kvm/mips.c b/arch/mips/kvm/mips.c
> > index a25e0b73ee70..45c7179144dc 100644
> > --- a/arch/mips/kvm/mips.c
> > +++ b/arch/mips/kvm/mips.c
> > @@ -387,6 +387,8 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
> > if (err)
> > goto out_free_gebase;
> >
> > + vcpu->arch.mmu_page_cache.capacity = KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE;
> > +
> > return 0;
> >
> > out_free_gebase:
> > diff --git a/arch/riscv/kvm/mmu.c b/arch/riscv/kvm/mmu.c
> > index f80a34fbf102..8c2338ecc246 100644
> > --- a/arch/riscv/kvm/mmu.c
> > +++ b/arch/riscv/kvm/mmu.c
> > @@ -347,10 +347,10 @@ 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 = {
> > + .capacity = KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE,
> > + .gfp_zero = __GFP_ZERO,
> > + };
> >
> > end = (gpa + size + PAGE_SIZE - 1) & PAGE_MASK;
> > pfn = __phys_to_pfn(hpa);
> > diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
> > index 6785aef4cbd4..bbcb9d4a04fb 100644
> > --- a/arch/riscv/kvm/vcpu.c
> > +++ b/arch/riscv/kvm/vcpu.c
> > @@ -94,6 +94,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
> >
> > /* Mark this VCPU never ran */
> > vcpu->arch.ran_atleast_once = false;
> > + vcpu->arch.mmu_page_cache.capacity = KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE;
> > vcpu->arch.mmu_page_cache.gfp_zero = __GFP_ZERO;
> >
> > /* Setup ISA features available to VCPU */
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 4b40fa2e27eb..dad7e19ef8ed 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -5803,12 +5803,21 @@ int kvm_mmu_create(struct kvm_vcpu *vcpu)
> > {
> > int ret;
> >
> > + vcpu->arch.mmu_pte_list_desc_cache.capacity =
> > + KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE;
> > vcpu->arch.mmu_pte_list_desc_cache.kmem_cache = pte_list_desc_cache;
> > vcpu->arch.mmu_pte_list_desc_cache.gfp_zero = __GFP_ZERO;
> >
> > + vcpu->arch.mmu_page_header_cache.capacity =
> > + KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE;
> > vcpu->arch.mmu_page_header_cache.kmem_cache = mmu_page_header_cache;
> > vcpu->arch.mmu_page_header_cache.gfp_zero = __GFP_ZERO;
> >
> > + vcpu->arch.mmu_shadowed_info_cache.capacity =
> > + KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE;
> > +
> > + vcpu->arch.mmu_shadow_page_cache.capacity =
> > + KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE;
> > vcpu->arch.mmu_shadow_page_cache.gfp_zero = __GFP_ZERO;
> >
> > vcpu->arch.mmu = &vcpu->arch.root_mmu;
> > diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
> > index ac1ebb37a0ff..549103a4f7bc 100644
> > --- a/include/linux/kvm_types.h
> > +++ b/include/linux/kvm_types.h
> > @@ -83,14 +83,19 @@ 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 object pointers is allocated dynamically when the
> > + * cache is topped-up. The capacity field defines the number of object pointers
> > + * available after the struct.
> > */
> > 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;
> > };
> > -#endif
> > +#endif /* KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE */
> >
> > #define HALT_POLL_HIST_COUNT 32
> >
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index e089db822c12..264e4107e06f 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -371,12 +371,23 @@ static inline void *mmu_memory_cache_alloc_obj(struct kvm_mmu_memory_cache *mc,
> >
> > int kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, 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 (WARN_ON(mc->capacity == 0))
> > + return -EINVAL;
> > +
> > + if (!mc->objects) {
> > + mc->objects = kvmalloc_array(sizeof(void *), mc->capacity, gfp);
> > + if (!mc->objects)
> > + return -ENOMEM;
> > + }
> > +
> > + 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;
> > @@ -397,6 +408,11 @@ void kvm_mmu_free_memory_cache(struct kvm_mmu_memory_cache *mc)
> > else
> > free_page((unsigned long)mc->objects[--mc->nobjs]);
> > }
> > +
> > + kvfree(mc->objects);
> > +
> > + /* Note, must set to NULL to avoid use-after-free in the next top-up. */
> > + mc->objects = NULL;
> > }
> >
> > void *kvm_mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc)
> > --
> > 2.36.0.550.gb090851708-goog
> >