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SubjectRe: [PATCH v5 0/7] Lazily allocate memslot rmaps
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On 18/05/21 19:34, 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.
>
> Changelog:
> v2:
> Incorporated feedback from Paolo and Sean
> Replaced the memslot_assignment_lock with slots_arch_lock, which
> has a larger critical section.
>
> v3:
> Removed shadow_mmu_active as suggested by Sean
> Removed everything except adding a return value to
> kvm_mmu_init_tdp_mmu from patch 1 of v2
> Added RCU protection and better memory ordering for installing the
> memslot rmaps as suggested by Paolo
> Reordered most of the patches
>
> v4:
> Renamed functions to allocate and free memslots based on feedback
> from David.
> Eliminated the goto in memslot_rmap_alloc, as David suggested.
> Eliminated kvm_memslots_have_rmaps and updated comments on uses of
> memslots_have_rmaps. Suggested by Paolo.
> Changed the description on patch 7 to one Paolo suggested.
> Collected Reviewed-by tags from David.
> Dropped the patch to add RCU notations to rmap accesses.
>
> v5:
> Responding to comments from Sean.
> Improved comments
> Swapped args to kvm_copy_memslots to match memcpy
> Fixed some line wrap and declaration style issues
> No longer check if memslots have rmaps before operations which
> iterate through active_mmu_pages
> Re-added the kvm_memslots_have_rmaps helper
> Fixed a couple missing unlocks for the slots_arch_lock

Queued (with minor conflicts), thanks!

Paolo

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