[PATCH v2 2/3] KVM: arm64: Align the pKVM selftest VM's stage-2 PGD

[Fuad Tabba <fuad.tabba@linux.dev>]

With CONFIG_NVHE_EL2_DEBUG, pKVM init sets up a selftest VM whose PGD is
the start of its 32-page block. That block is the first allocation out
of the hyp reserve, which falls back to page alignment when its
PMD-aligned allocation fails (512M on a 64K host).

A PGD of concatenated tables must be naturally aligned, or the VM's pool
never coalesces it into a block hyp_alloc_pages() can return. So when
the reserve isn't aligned to the PGD's size, the allocation in
guest_s2_zalloc_pages_exact() fails and pKVM init panics. A 64K host
with a 44-bit PA has a four-page PGD, and on a 1G QEMU VM the reserve
landed at 0x7fae0000.

Take the aligned pages inside the block instead. The pages either side
of it go to the memcache and the pool. The PGD is at most 16 pages, so
it always fits.

Fixes: 48d5645072930 ("KVM: arm64: Extend pKVM selftest for np-guests")
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---
 arch/arm64/kvm/hyp/nvhe/pkvm.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/hyp/nvhe/pkvm.c b/arch/arm64/kvm/hyp/nvhe/pkvm.c
index 03e530e15b4a2..820baf9ff74a2 100644
--- a/arch/arm64/kvm/hyp/nvhe/pkvm.c
+++ b/arch/arm64/kvm/hyp/nvhe/pkvm.c
@@ -847,10 +847,14 @@ struct pkvm_hyp_vcpu *init_selftest_vm(void *virt)
 {
 	struct hyp_page *p = hyp_virt_to_page(virt);
 	unsigned long min_pages, seeded = 0;
+	void *pgd;
 	int i;
 
 	selftest_vm.kvm.arch.mmu.vtcr = host_mmu.arch.mmu.vtcr;
-	WARN_ON(kvm_guest_prepare_stage2(&selftest_vm, virt));
+
+	/* A concatenated PGD must be naturally aligned for the pool to coalesce it. */
+	pgd = PTR_ALIGN(virt, kvm_pgtable_stage2_pgd_size(selftest_vm.kvm.arch.mmu.vtcr));
+	WARN_ON(kvm_guest_prepare_stage2(&selftest_vm, pgd));
 
 	/*
 	 * Mirror pkvm_refill_memcache() for the share/donate pre-checks;
-- 
2.47.3