KVM: arm64: Pre-check vcpu memcache for host->guest donate

KVM: arm64: Pre-check vcpu memcache for host->guest donate

__pkvm_host_donate_guest() flips the host stage-2 PTE for the
donated page to a non-valid annotation via
host_stage2_set_owner_metadata_locked() and then calls
kvm_pgtable_stage2_map() to install the matching guest stage-2
mapping. The map's return value is wrapped in WARN_ON() and
otherwise discarded, asserting that the call cannot fail.

WARN_ON() at nVHE EL2 panics, so this assertion is only correct
if the call genuinely cannot fail. kvm_pgtable_stage2_map() can
fail with -ENOMEM even at PAGE_SIZE granularity: the donate path
verifies PKVM_NOPAGE for the guest IPA before the map, so the
walker must allocate fresh page-table pages from the vcpu
memcache, and the host controls the vcpu memcache via the topup
interface. An under-provisioned donation request would otherwise
turn a recoverable -ENOMEM into a fatal hyp panic.

Bound the worst-case walker allocation alongside the existing
__host_check_page_state_range() / __guest_check_page_state_range()
pre-checks, using the helper introduced for host->guest share. If
the vcpu memcache holds fewer pages than kvm_mmu_cache_min_pages(),
return -ENOMEM before any state mutation.

Fixes: 1e579adca177 ("KVM: arm64: Introduce __pkvm_host_donate_guest()")
Assisted-by: Gemini:gemini-3.1-pro review-prompts
Signed-off-by: Fuad Tabba <tabba@google.com>
Link: https://patch.msgid.link/20260501112149.2824881-7-tabba@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>