Re: [PATCH v2 0/7] Lazily allocate memslot rmaps

From: Paolo Bonzini
Date: Mon May 03 2021 - 09:45:06 EST

On 29/04/21 23:18, Ben Gardon wrote:
This series enables KVM to save memory when using the TDP MMU by waiting
to allocate memslot rmaps until they are needed. To do this, KVM tracks
whether or not a shadow root has been allocated. In order to get away
with not allocating the rmaps, KVM must also be sure to skip operations
which iterate over the rmaps. If the TDP MMU is in use and we have not
allocated a shadow root, these operations would essentially be op-ops
anyway. Skipping the rmap operations has a secondary benefit of avoiding
acquiring the MMU lock in write mode in many cases, substantially
reducing MMU lock contention.

This series was tested on an Intel Skylake machine. With the TDP MMU off
and on, this introduced no new failures on kvm-unit-tests or KVM selftests.

Thanks, I only reported some technicalities in the ordering of loads (which matter since the loads happen with SRCU protection only). Apart from this, this looks fine!


Incorporated feedback from Paolo and Sean
Replaced the memslot_assignment_lock with slots_arch_lock, which
has a larger critical section.

Ben Gardon (7):
KVM: x86/mmu: Track if shadow MMU active
KVM: x86/mmu: Skip rmap operations if shadow MMU inactive
KVM: x86/mmu: Deduplicate rmap freeing
KVM: x86/mmu: Factor out allocating memslot rmap
KVM: mmu: Refactor memslot copy
KVM: mmu: Add slots_arch_lock for memslot arch fields
KVM: x86/mmu: Lazily allocate memslot rmaps

arch/x86/include/asm/kvm_host.h | 13 +++
arch/x86/kvm/mmu/mmu.c | 153 +++++++++++++++++++++-----------
arch/x86/kvm/mmu/mmu_internal.h | 2 +
arch/x86/kvm/mmu/tdp_mmu.c | 6 +-
arch/x86/kvm/mmu/tdp_mmu.h | 4 +-
arch/x86/kvm/x86.c | 110 +++++++++++++++++++----
include/linux/kvm_host.h | 9 ++
virt/kvm/kvm_main.c | 54 ++++++++---
8 files changed, 264 insertions(+), 87 deletions(-)