KVM: x86/mmu: Zap only SPs that shadow gPTEs when deleting memslot

When performing a targeted zap on memslot removal, zap only MMU pages that
shadow guest PTEs, as zapping all SPs that "match" the gfn is inexact and
unnecessary.  Furthermore, for_each_gfn_valid_sp() arguably shouldn't
exist, because it doesn't do what most people would it expect it to do.
The "round gfn for level" adjustment that is done for direct SPs (no gPTE)
means that the exact gfn comparison will not get a match, even when a SP
does "cover" a gfn, or was even created specifically for a gfn.

For memslot deletion specifically, KVM's behavior will vary significantly
based on the size and alignment of a memslot, and in weird ways.  E.g. for
a 4KiB memslot, KVM will zap more SPs if the slot is 1GiB aligned than if
it's only 4KiB aligned.  And as described below, zapping SPs in the
aligned case overzaps for direct MMUs, as odds are good the upper-level
SPs are serving other memslots.

To iterate over all potentially-relevant gfns, KVM would need to make a
pass over the hash table for each level, with the gfn used for lookup
rounded for said level.  And then check that the SP is of the correct
level, too, e.g. to avoid over-zapping.

But even then, KVM would massively overzap, as processing every level is
all but guaranteed to zap SPs that serve other memslots, especially if the
memslot being removed is relatively small.  KVM could mitigate that issue
by processing only levels that can be possible guest huge pages, i.e. are
less likely to be re-used for other memslot, but while somewhat logical,
that's quite arbitrary and would be a bit of a mess to implement.

So, zap only SPs with gPTEs, as the resulting behavior is easy to describe,
is predictable, and is explicitly minimal, i.e. KVM only zaps SPs that
absolutely must be zapped.

Cc: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Yan Zhao <yan.y.zhao@intel.com>
Message-ID: <20241009192345.1148353-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>