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This together with the next patch will fix a future race between
kvm_zap_gfn_range and the page fault handler, which will happen
when AVIC memslot is going to be only partially disabled.

The performance impact is minimal since kvm_zap_gfn_range is only
called by users, update_mtrr() and kvm_post_set_cr0().

Both only use it if the guest has non-coherent DMA, in order to
honor the guest's UC memtype.

MTRR and CD setup only happens at boot, and generally in an area
where the page tables should be small (for CD) or should not
include the affected GFNs at all (for MTRRs).

This is based on a patch suggested by Sean Christopherson:
https://lkml.org/lkml/2021/7/22/1025

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210810205251.424103-2-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>