| Message ID | 20200605213853.14959-9-sean.j.christopherson@intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: Cleanup and unify kvm_mmu_memory_cache usage | expand |
On Fri, Jun 5, 2020 at 2:39 PM Sean Christopherson <sean.j.christopherson@intel.com> 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@intel.com> Reviewed-by: Ben Gardon <bgardon@google.com> > --- > 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 >
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)
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@intel.com> --- arch/x86/kvm/mmu/mmu.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-)