Split huge pages eagerly when enabling dirty logging. The goal is to
avoid doing it while faulting on write-protected pages, which
negatively impacts guest performance.

A memslot marked for dirty logging is split in 1GB pieces at a time.
This is in order to release the mmu_lock and give other kernel threads
the opportunity to run, and also in order to allocate enough pages to
split a 1GB range worth of huge pages (or a single 1GB huge page).
Note that these page allocations can fail, so eager page splitting is
best-effort.  This is not a correctness issue though, as huge pages
can still be split on write-faults.

Eager page splitting only takes effect when the huge page mapping has
been existing in the stage-2 page table. Otherwise, the huge page will
be mapped to multiple non-huge pages on page fault.

The benefits of eager page splitting are the same as in x86, added
with commit a3fe5dbd ("KVM: x86/mmu: Split huge pages mapped by
the TDP MMU when dirty logging is enabled"). For example, when running
dirty_log_perf_test with 64 virtual CPUs (Ampere Altra), 1GB per vCPU,
50% reads, and 2MB HugeTLB memory, the time it takes vCPUs to access
all of their memory after dirty logging is enabled decreased by 44%
from 2.58s to 1.42s.
Signed-off-by: Ricardo Koller <ricarkol@google.com>
Reviewed-by: Shaoqin Huang <shahuang@redhat.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Link: https://lore.kernel.org/r/20230426172330.1439644-10-ricarkol@google.comSigned-off-by: Oliver Upton <oliver.upton@linux.dev>