Re: [PATCH gmem] KVM: selftests: Fix gmem conversion tests for multiple vCPUs

[Sean Christopherson]

On Mon, Oct 16, 2023, Michael Roth wrote:
> Currently the private_mem_conversions_test crashes if invoked with the
> -n <num_vcpus> option without also specifying multiple memslots via -m.

Totally a PEBKAC, not a bug ;-)
 
> This is because the current implementation assumes -m is specified and
> always sets up the per-vCPU memory with a dedicated memslot for each
> vCPU. When -m is not specified, the test skips setting up
> memslots/memory for secondary vCPUs.
> 
> The current code does seem to try to handle using a single memslot for
> multiple vCPUs in some places, e.g. the call-site, but
> test_mem_conversions() is missing the important bit of sizing the single
> memslot appropriately to handle all the per-vCPU memory. Implement that
> handling.
> 
> Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
> ---
>  .../kvm/x86_64/private_mem_conversions_test.c        | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/x86_64/private_mem_conversions_test.c b/tools/testing/selftests/kvm/x86_64/private_mem_conversions_test.c
> index c04e7d61a585..5eb693fead33 100644
> --- a/tools/testing/selftests/kvm/x86_64/private_mem_conversions_test.c
> +++ b/tools/testing/selftests/kvm/x86_64/private_mem_conversions_test.c
> @@ -388,10 +388,14 @@ static void test_mem_conversions(enum vm_mem_backing_src_type src_type, uint32_t
>  		gmem_flags = 0;
>  	memfd = vm_create_guest_memfd(vm, memfd_size, gmem_flags);
>  
> -	for (i = 0; i < nr_memslots; i++)
> -		vm_mem_add(vm, src_type, BASE_DATA_GPA + size * i,
> -			   BASE_DATA_SLOT + i, size / vm->page_size,
> -			   KVM_MEM_PRIVATE, memfd, size * i);
> +	if (nr_memslots == 1)
> +		vm_mem_add(vm, src_type, BASE_DATA_GPA, BASE_DATA_SLOT,
> +			   memfd_size / vm->page_size, KVM_MEM_PRIVATE, memfd, 0);
> +	else
> +		for (i = 0; i < nr_memslots; i++)

The if-else needs curly braces.

> +			vm_mem_add(vm, src_type, BASE_DATA_GPA + size * i,
> +				   BASE_DATA_SLOT + i, size / vm->page_size,
> +				   KVM_MEM_PRIVATE, memfd, size * i);

But I think that's a moot point, because isn't it easier to do this?

diff --git a/tools/testing/selftests/kvm/x86_64/private_mem_conversions_test.c b/tools/testing/selftests/kvm/x86_64/private_mem_conversions_test.c
index c04e7d61a585..c99073098f98 100644
--- a/tools/testing/selftests/kvm/x86_64/private_mem_conversions_test.c
+++ b/tools/testing/selftests/kvm/x86_64/private_mem_conversions_test.c
@@ -367,6 +367,7 @@ static void test_mem_conversions(enum vm_mem_backing_src_type src_type, uint32_t
         */
        const size_t size = align_up(PER_CPU_DATA_SIZE, get_backing_src_pagesz(src_type));
        const size_t memfd_size = size * nr_vcpus;
+       const size_t slot_size = memfd_size / nr_memslots;
        struct kvm_vcpu *vcpus[KVM_MAX_VCPUS];
        pthread_t threads[KVM_MAX_VCPUS];
        uint64_t gmem_flags;
@@ -390,7 +391,7 @@ static void test_mem_conversions(enum vm_mem_backing_src_type src_type, uint32_t
 
        for (i = 0; i < nr_memslots; i++)
                vm_mem_add(vm, src_type, BASE_DATA_GPA + size * i,
-                          BASE_DATA_SLOT + i, size / vm->page_size,
+                          BASE_DATA_SLOT + i, slot_size / vm->page_size,
                           KVM_MEM_PRIVATE, memfd, size * i);
 
        for (i = 0; i < nr_vcpus; i++) {