KVM: selftests: Add additional pages to the guest to accommodate ucall

Add additional pages to the guest to account for the number of pages
the ucall headers need.  The only reason things worked before is the
ucall headers are fairly small.  If they were ever to increase in
size the guest could run out of memory.

This is done in preparation for adding string formatting options to
the guest through the ucall framework which increases the size of
the ucall headers.

Fixes: 426729b ("KVM: selftests: Add ucall pool based implementation")
Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Link: https://lore.kernel.org/r/20230729003643.1053367-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: suomilewis

tools/testing/selftests/kvm/include/ucall_common.h

@@ -34,6 +34,7 @@ void *ucall_arch_get_ucall(struct kvm_vcpu *vcpu);
 void ucall(uint64_t cmd, int nargs, ...);
 uint64_t get_ucall(struct kvm_vcpu *vcpu, struct ucall *uc);
 void ucall_init(struct kvm_vm *vm, vm_paddr_t mmio_gpa);
+int ucall_nr_pages_required(uint64_t page_size);
 
 /*
  * Perform userspace call without any associated data.  This bare call avoids

tools/testing/selftests/kvm/lib/kvm_util.c

@@ -312,6 +312,7 @@ static uint64_t vm_nr_pages_required(enum vm_guest_mode mode,
 				     uint32_t nr_runnable_vcpus,
 				     uint64_t extra_mem_pages)
 {
+	uint64_t page_size = vm_guest_mode_params[mode].page_size;
 	uint64_t nr_pages;
 
 	TEST_ASSERT(nr_runnable_vcpus,
@@ -340,6 +341,9 @@ static uint64_t vm_nr_pages_required(enum vm_guest_mode mode,
 	 */
 	nr_pages += (nr_pages + extra_mem_pages) / PTES_PER_MIN_PAGE * 2;
 
+	/* Account for the number of pages needed by ucall. */
+	nr_pages += ucall_nr_pages_required(page_size);
+
 	return vm_adjust_num_guest_pages(mode, nr_pages);
 }
 

tools/testing/selftests/kvm/lib/ucall_common.c

@@ -11,6 +11,11 @@ struct ucall_header {
 	struct ucall ucalls[KVM_MAX_VCPUS];
 };
 
+int ucall_nr_pages_required(uint64_t page_size)
+{
+	return align_up(sizeof(struct ucall_header), page_size) / page_size;
+}
+
 /*
  * ucall_pool holds per-VM values (global data is duplicated by each VM), it
  * must not be accessed from host code.