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

Paolo

> Changelog:
> v2:
> 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(-)
>

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