Re: [PATCH v2 00/21] KVM: Cleanup and unify kvm_mmu_memory_cache usage

Re: [PATCH v2 00/21] KVM: Cleanup and unify kvm_mmu_memory_cache usage

[Sean Christopherson]

On Mon, Jun 22, 2020 at 01:08:01PM -0700, Sean Christopherson wrote:
> Note, patch 18 will conflict with the p4d rework in 5.8.  I originally
> stated I would send v2 only after that got pulled into Paolo's tree, but
> I got my timing wrong, i.e. I was thinking that would have already
> happened.  I'll send v3 if necessary.  I wanted to get v2 out there now
> that I actually compile tested other architectures.

Gah, too impatient by one day :-)  I'll spin v3 later in the week.

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Re: [PATCH v2 05/21] KVM: x86/mmu: Try to avoid crashing KVM if a MMU memory cache is empty

[Ben Gardon]

On Mon, Jun 22, 2020 at 1:09 PM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> Attempt to allocate a new object instead of crashing KVM (and likely the
> kernel) if a memory cache is unexpectedly empty.  Use GFP_ATOMIC for the
> allocation as the caches are used while holding mmu_lock.  The immediate
> BUG_ON() makes the code unnecessarily explosive and led to confusing
> minimums being used in the past, e.g. allocating 4 objects where 1 would
> suffice.
>
Reviewed-by: Ben Gardon <bgardon@google.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 21 +++++++++++++++------
>  1 file changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index ba70de24a5b0..5e773564ab20 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -1060,6 +1060,15 @@ static void walk_shadow_page_lockless_end(struct kvm_vcpu *vcpu)
>         local_irq_enable();
>  }
>
> +static inline void *mmu_memory_cache_alloc_obj(struct kvm_mmu_memory_cache *mc,
> +                                              gfp_t gfp_flags)
> +{
> +       if (mc->kmem_cache)
> +               return kmem_cache_zalloc(mc->kmem_cache, gfp_flags);
> +       else
> +               return (void *)__get_free_page(gfp_flags);
> +}
> +
>  static int mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int min)
>  {
>         void *obj;
> @@ -1067,10 +1076,7 @@ static int mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int min)
>         if (mc->nobjs >= min)
>                 return 0;
>         while (mc->nobjs < ARRAY_SIZE(mc->objects)) {
> -               if (mc->kmem_cache)
> -                       obj = kmem_cache_zalloc(mc->kmem_cache, GFP_KERNEL_ACCOUNT);
> -               else
> -                       obj = (void *)__get_free_page(GFP_KERNEL_ACCOUNT);
> +               obj = mmu_memory_cache_alloc_obj(mc, GFP_KERNEL_ACCOUNT);
>                 if (!obj)
>                         return mc->nobjs >= min ? 0 : -ENOMEM;
>                 mc->objects[mc->nobjs++] = obj;
> @@ -1118,8 +1124,11 @@ static void *mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc)
>  {
>         void *p;
>
> -       BUG_ON(!mc->nobjs);
> -       p = mc->objects[--mc->nobjs];
> +       if (WARN_ON(!mc->nobjs))
> +               p = mmu_memory_cache_alloc_obj(mc, GFP_ATOMIC | __GFP_ACCOUNT);
> +       else
> +               p = mc->objects[--mc->nobjs];
> +       BUG_ON(!p);
>         return p;
>  }
>
> --
> 2.26.0
>

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Re: [PATCH v2 14/21] KVM: Move x86's version of struct kvm_mmu_memory_cache to common code

[Ben Gardon]

On Mon, Jun 22, 2020 at 1:09 PM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> Move x86's 'struct kvm_mmu_memory_cache' to common code in anticipation
> of moving the entire x86 implementation code to common KVM and reusing
> it for arm64 and MIPS.  Add a new architecture specific asm/kvm_types.h
> to control the existence and parameters of the struct.  The new header
> is needed to avoid a chicken-and-egg problem with asm/kvm_host.h as all
> architectures define instances of the struct in their vCPU structs.
>
> Add an asm-generic version of kvm_types.h to avoid having empty files on
> PPC and s390 in the long term, and for arm64 and mips in the short term.
>
> Suggested-by: Christoffer Dall <christoffer.dall@arm.com>
Reviewed-by: Ben Gardon <bgardon@google.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
>  arch/arm64/include/asm/Kbuild    |  1 +
>  arch/mips/include/asm/Kbuild     |  1 +
>  arch/powerpc/include/asm/Kbuild  |  1 +
>  arch/s390/include/asm/Kbuild     |  1 +
>  arch/x86/include/asm/kvm_host.h  | 13 -------------
>  arch/x86/include/asm/kvm_types.h |  7 +++++++
>  include/asm-generic/kvm_types.h  |  5 +++++
>  include/linux/kvm_types.h        | 19 +++++++++++++++++++
>  8 files changed, 35 insertions(+), 13 deletions(-)
>  create mode 100644 arch/x86/include/asm/kvm_types.h
>  create mode 100644 include/asm-generic/kvm_types.h
>
> diff --git a/arch/arm64/include/asm/Kbuild b/arch/arm64/include/asm/Kbuild
> index ff9cbb631212..35a68155cd0e 100644
> --- a/arch/arm64/include/asm/Kbuild
> +++ b/arch/arm64/include/asm/Kbuild
> @@ -1,5 +1,6 @@
>  # SPDX-License-Identifier: GPL-2.0
>  generic-y += early_ioremap.h
> +generic-y += kvm_types.h
>  generic-y += local64.h
>  generic-y += mcs_spinlock.h
>  generic-y += qrwlock.h
> diff --git a/arch/mips/include/asm/Kbuild b/arch/mips/include/asm/Kbuild
> index 8643d313890e..397e6d24d2ab 100644
> --- a/arch/mips/include/asm/Kbuild
> +++ b/arch/mips/include/asm/Kbuild
> @@ -5,6 +5,7 @@ generated-y += syscall_table_64_n32.h
>  generated-y += syscall_table_64_n64.h
>  generated-y += syscall_table_64_o32.h
>  generic-y += export.h
> +generic-y += kvm_types.h
>  generic-y += local64.h
>  generic-y += mcs_spinlock.h
>  generic-y += parport.h
> diff --git a/arch/powerpc/include/asm/Kbuild b/arch/powerpc/include/asm/Kbuild
> index dadbcf3a0b1e..2d444d09b553 100644
> --- a/arch/powerpc/include/asm/Kbuild
> +++ b/arch/powerpc/include/asm/Kbuild
> @@ -4,6 +4,7 @@ generated-y += syscall_table_64.h
>  generated-y += syscall_table_c32.h
>  generated-y += syscall_table_spu.h
>  generic-y += export.h
> +generic-y += kvm_types.h
>  generic-y += local64.h
>  generic-y += mcs_spinlock.h
>  generic-y += vtime.h
> diff --git a/arch/s390/include/asm/Kbuild b/arch/s390/include/asm/Kbuild
> index 83f6e85de7bc..319efa0e6d02 100644
> --- a/arch/s390/include/asm/Kbuild
> +++ b/arch/s390/include/asm/Kbuild
> @@ -6,5 +6,6 @@ generated-y += unistd_nr.h
>
>  generic-y += asm-offsets.h
>  generic-y += export.h
> +generic-y += kvm_types.h
>  generic-y += local64.h
>  generic-y += mcs_spinlock.h
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 67b84aa2984e..70832aa762e5 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -193,8 +193,6 @@ struct x86_exception;
>  enum x86_intercept;
>  enum x86_intercept_stage;
>
> -#define KVM_NR_MEM_OBJS 40
> -
>  #define KVM_NR_DB_REGS 4
>
>  #define DR6_BD         (1 << 13)
> @@ -245,17 +243,6 @@ enum x86_intercept_stage;
>
>  struct kvm_kernel_irq_routing_entry;
>
> -/*
> - * We don't want allocation failures within the mmu code, so we preallocate
> - * enough memory for a single page fault in a cache.
> - */
> -struct kvm_mmu_memory_cache {
> -       int nobjs;
> -       gfp_t gfp_zero;
> -       struct kmem_cache *kmem_cache;
> -       void *objects[KVM_NR_MEM_OBJS];
> -};
> -
>  /*
>   * the pages used as guest page table on soft mmu are tracked by
>   * kvm_memory_slot.arch.gfn_track which is 16 bits, so the role bits used
> diff --git a/arch/x86/include/asm/kvm_types.h b/arch/x86/include/asm/kvm_types.h
> new file mode 100644
> index 000000000000..08f1b57d3b62
> --- /dev/null
> +++ b/arch/x86/include/asm/kvm_types.h
> @@ -0,0 +1,7 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_X86_KVM_TYPES_H
> +#define _ASM_X86_KVM_TYPES_H
> +
> +#define KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE 40
> +
> +#endif /* _ASM_X86_KVM_TYPES_H */
> diff --git a/include/asm-generic/kvm_types.h b/include/asm-generic/kvm_types.h
> new file mode 100644
> index 000000000000..2a82daf110f1
> --- /dev/null
> +++ b/include/asm-generic/kvm_types.h
> @@ -0,0 +1,5 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_GENERIC_KVM_TYPES_H
> +#define _ASM_GENERIC_KVM_TYPES_H
> +
> +#endif
> diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
> index 68e84cf42a3f..a7580f69dda0 100644
> --- a/include/linux/kvm_types.h
> +++ b/include/linux/kvm_types.h
> @@ -20,6 +20,8 @@ enum kvm_mr_change;
>
>  #include <linux/types.h>
>
> +#include <asm/kvm_types.h>
> +
>  /*
>   * Address types:
>   *
> @@ -58,4 +60,21 @@ struct gfn_to_pfn_cache {
>         bool dirty;
>  };
>
> +#ifdef KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE
> +/*
> + * Memory caches are used to preallocate memory ahead of various MMU flows,
> + * e.g. page fault handlers.  Gracefully handling allocation failures deep in
> + * 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.
> + */
> +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];
> +};
> +#endif
> +
> +
>  #endif /* __KVM_TYPES_H__ */
> --
> 2.26.0
>

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