Re: [PATCH 2/4] KVM: selftests: Setup ucall after loading program into guest memory

[Sean Christopherson]

On Wed, Dec 07, 2022, Oliver Upton wrote:
> The new ucall infrastructure needs to update a couple of guest globals
> to pass through the ucall MMIO addr and pool of ucall structs. A
> precondition of this actually working is to have the program image
> already loaded into guest memory.

Ouch.  Might be worth explicitly stating what goes wrong.  Even though it's super
obvious in hindsight, it still took me a few seconds to understand what
precondition you were referring to, e.g. I was trying to figure out how selecting
the MMIO address depended on the guest code being loaded...

> 
> Call ucall_init() after kvm_vm_elf_load(). Continue to park the ucall
> MMIO addr after MEM_REGION_TEST_DATA.
> 
> Signed-off-by: Oliver Upton <oliver.upton@xxxxxxxxx>
> ---
>  tools/testing/selftests/kvm/aarch64/page_fault_test.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/kvm/aarch64/page_fault_test.c b/tools/testing/selftests/kvm/aarch64/page_fault_test.c
> index 92d3a91153b6..95d22cfb7b41 100644
> --- a/tools/testing/selftests/kvm/aarch64/page_fault_test.c
> +++ b/tools/testing/selftests/kvm/aarch64/page_fault_test.c
> @@ -609,8 +609,13 @@ static void setup_memslots(struct kvm_vm *vm, struct test_params *p)
>  				    data_size / guest_page_size,
>  				    p->test_desc->data_memslot_flags);
>  	vm->memslots[MEM_REGION_TEST_DATA] = TEST_DATA_MEMSLOT;
> +}
> +
> +static void setup_ucall(struct kvm_vm *vm)
> +{
> +	struct userspace_mem_region *region = vm_get_mem_region(vm, MEM_REGION_TEST_DATA);
>  
> -	ucall_init(vm, data_gpa + data_size);
> +	ucall_init(vm, region->region.guest_phys_addr + region->region.memory_size);

Isn't there a hole after CODE_AND_DATA_MEMSLOT?  I.e. after memslot 0?  The reason
I ask is because if so, then we can do the temporarily heinous, but hopefully forward
looking thing of adding a helper to wrap kvm_vm_elf_load() + ucall_init().

E.g. I think we can do this immediately, and then at some point in the 6.2 cycle
add a dedicated region+memslot for the ucall MMIO page.

---
 .../selftests/kvm/aarch64/page_fault_test.c   | 10 +------
 .../selftests/kvm/include/kvm_util_base.h     |  1 +
 tools/testing/selftests/kvm/lib/kvm_util.c    | 28 +++++++++++--------
 3 files changed, 19 insertions(+), 20 deletions(-)

diff --git a/tools/testing/selftests/kvm/aarch64/page_fault_test.c b/tools/testing/selftests/kvm/aarch64/page_fault_test.c
index 95d22cfb7b41..68c47db2eb2e 100644
--- a/tools/testing/selftests/kvm/aarch64/page_fault_test.c
+++ b/tools/testing/selftests/kvm/aarch64/page_fault_test.c
@@ -611,13 +611,6 @@ static void setup_memslots(struct kvm_vm *vm, struct test_params *p)
 	vm->memslots[MEM_REGION_TEST_DATA] = TEST_DATA_MEMSLOT;
 }
 
-static void setup_ucall(struct kvm_vm *vm)
-{
-	struct userspace_mem_region *region = vm_get_mem_region(vm, MEM_REGION_TEST_DATA);
-
-	ucall_init(vm, region->region.guest_phys_addr + region->region.memory_size);
-}
-
 static void setup_default_handlers(struct test_desc *test)
 {
 	if (!test->mmio_handler)
@@ -706,8 +699,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)
 
 	vm = ____vm_create(mode);
 	setup_memslots(vm, p);
-	kvm_vm_elf_load(vm, program_invocation_name);
-	setup_ucall(vm);
+	vm_init_guest_code_and_data(vm);
 	vcpu = vm_vcpu_add(vm, 0, guest_code);
 
 	setup_gva_maps(vm);
diff --git a/tools/testing/selftests/kvm/include/kvm_util_base.h b/tools/testing/selftests/kvm/include/kvm_util_base.h
index fbc2a79369b8..175b5ca0c061 100644
--- a/tools/testing/selftests/kvm/include/kvm_util_base.h
+++ b/tools/testing/selftests/kvm/include/kvm_util_base.h
@@ -682,6 +682,7 @@ vm_paddr_t vm_phy_page_alloc(struct kvm_vm *vm, vm_paddr_t paddr_min,
 vm_paddr_t vm_phy_pages_alloc(struct kvm_vm *vm, size_t num,
 			      vm_paddr_t paddr_min, uint32_t memslot);
 vm_paddr_t vm_alloc_page_table(struct kvm_vm *vm);
+void vm_init_guest_code_and_data(struct kvm_vm *vm);
 
 /*
  * ____vm_create() does KVM_CREATE_VM and little else.  __vm_create() also
diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
index c88c3ace16d2..0eab6b11a6e9 100644
--- a/tools/testing/selftests/kvm/lib/kvm_util.c
+++ b/tools/testing/selftests/kvm/lib/kvm_util.c
@@ -329,12 +329,27 @@ static uint64_t vm_nr_pages_required(enum vm_guest_mode mode,
 	return vm_adjust_num_guest_pages(mode, nr_pages);
 }
 
+void vm_init_guest_code_and_data(struct kvm_vm *vm)
+{
+	struct userspace_mem_region *slot;
+
+	kvm_vm_elf_load(vm, program_invocation_name);
+
+	/*
+	 * TODO: Add a dedicated memory region to carve out MMIO.  KVM treats
+	 * writes to read-only memslots as MMIO, and creating a read-only
+	 * memslot for the MMIO region would prevent silently clobbering the
+	 * MMIO region.
+	 */
+	slot = vm_get_mem_region(vm, MEM_REGION_DATA);
+	ucall_init(vm, slot->region.guest_phys_addr + slot->region.memory_size);
+}
+
 struct kvm_vm *__vm_create(enum vm_guest_mode mode, uint32_t nr_runnable_vcpus,
 			   uint64_t nr_extra_pages)
 {
 	uint64_t nr_pages = vm_nr_pages_required(mode, nr_runnable_vcpus,
 						 nr_extra_pages);
-	struct userspace_mem_region *slot0;
 	struct kvm_vm *vm;
 	int i;
 
@@ -347,16 +362,7 @@ struct kvm_vm *__vm_create(enum vm_guest_mode mode, uint32_t nr_runnable_vcpus,
 	for (i = 0; i < NR_MEM_REGIONS; i++)
 		vm->memslots[i] = 0;
 
-	kvm_vm_elf_load(vm, program_invocation_name);
-
-	/*
-	 * TODO: Add proper defines to protect the library's memslots, and then
-	 * carve out memslot1 for the ucall MMIO address.  KVM treats writes to
-	 * read-only memslots as MMIO, and creating a read-only memslot for the
-	 * MMIO region would prevent silently clobbering the MMIO region.
-	 */
-	slot0 = memslot2region(vm, 0);
-	ucall_init(vm, slot0->region.guest_phys_addr + slot0->region.memory_size);
+	vm_init_guest_code_and_data(vm);
 
 	kvm_arch_vm_post_create(vm);
 

base-commit: 1cf369f929d607760bf721f3eb9e965ed9c703e3
--