Re: [PATCH 08/21] KVM: x86/mmu: Clean up the gorilla math in mmu_topup_memory_caches()

[Ben Gardon]

On Fri, Jun 5, 2020 at 2:39 PM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> Clean up the minimums in mmu_topup_memory_caches() to document the
> driving mechanisms behind the minimums.  Now that encountering an empty
> cache is unlikely to trigger BUG_ON(), it is less dangerous to be more
> precise when defining the minimums.
>
> For rmaps, the logic is 1 parent PTE per level, plus a single rmap, and
> prefetched rmaps.  The extra objects in the current '8 + PREFETCH'
> minimum came about due to an abundance of paranoia in commit
> c41ef344de212 ("KVM: MMU: increase per-vcpu rmap cache alloc size"),
> i.e. it could have increased the minimum to 2 rmaps.  Furthermore, the
> unexpected extra rmap case was killed off entirely by commits
> f759e2b4c728c ("KVM: MMU: avoid pte_list_desc running out in
> kvm_mmu_pte_write") and f5a1e9f89504f ("KVM: MMU: remove call to
> kvm_mmu_pte_write from walk_addr").
>
> For the so called page cache, replace '8' with 2*PT64_ROOT_MAX_LEVEL.
> The 2x multiplier is needed because the cache is used for both shadow
> pages and gfn arrays for indirect MMUs.
>
> And finally, for page headers, replace '4' with PT64_ROOT_MAX_LEVEL.
>
> Note, KVM now supports 5-level paging, i.e. the old minimums that used a
> baseline derived from 4-level paging were technically wrong.  But, KVM
> always allocates roots in a separate flow, e.g. it's impossible in the
> current implementation to actually need 5 new shadow pages in a single
> flow.  Use PT64_ROOT_MAX_LEVEL unmodified instead of subtracting 1, as
> the direct usage is likely more intuitive to uninformed readers, and the
> inflated minimum is unlikely to affect functionality in practice.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Reviewed-by: Ben Gardon <bgardon@xxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/mmu.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 4b4c3234d623..451e0365e5dd 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -1103,14 +1103,17 @@ static int mmu_topup_memory_caches(struct kvm_vcpu *vcpu)
>  {
>         int r;
>
> +       /* 1 rmap, 1 parent PTE per level, and the prefetched rmaps. */
>         r = mmu_topup_memory_cache(&vcpu->arch.mmu_pte_list_desc_cache,
> -                                  8 + PTE_PREFETCH_NUM);
> +                                  1 + PT64_ROOT_MAX_LEVEL + PTE_PREFETCH_NUM);
>         if (r)
>                 return r;
> -       r = mmu_topup_memory_cache(&vcpu->arch.mmu_page_cache, 8);
> +       r = mmu_topup_memory_cache(&vcpu->arch.mmu_page_cache,
> +                                  2 * PT64_ROOT_MAX_LEVEL);
>         if (r)
>                 return r;
> -       return mmu_topup_memory_cache(&vcpu->arch.mmu_page_header_cache, 4);
> +       return mmu_topup_memory_cache(&vcpu->arch.mmu_page_header_cache,
> +                                     PT64_ROOT_MAX_LEVEL);
>  }
>
>  static void mmu_free_memory_caches(struct kvm_vcpu *vcpu)
> --
> 2.26.0
>