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KVM: x86: avoid unnecessary rmap walks when creating/moving slots

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On large memory guests it has been observed that creating a memslot
for a very large range can take noticeable amount of time.
Investigation showed that the time is spent walking the rmaps to update
existing sptes to remove write access or set/clear dirty bits to support
dirty logging.  These rmap walks are unnecessary when creating or moving
a memslot.  A newly created memslot will not have any existing mappings,
and the existing mappings of a moved memslot will have been invalidated
and flushed.  Any mappings established once the new/moved memslot becomes
visible will be set using the properties of the new slot.

Signed-off-by: Anthony Yznaga <anthony.yznaga@oracle.com>
Message-Id: <1591128450-11977-3-git-send-email-anthony.yznaga@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Anthony Yznaga 2020-06-02 13:07:29 -07:00 committed by Paolo Bonzini
parent 5688fed649
commit 4b44295538
1 changed files with 1 additions and 1 deletions
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2
arch/x86/kvm/x86.c
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@ -10242,7 +10242,7 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
*
* FIXME: const-ify all uses of struct kvm_memory_slot.
*/
if (change != KVM_MR_DELETE)
if (change == KVM_MR_FLAGS_ONLY)
kvm_mmu_slot_apply_flags(kvm, (struct kvm_memory_slot *) new);
/* Free the arrays associated with the old memslot. */