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

Sean Christopherson
2020-06-05T21:43:27Z

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(-)

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