Re: [PATCH] selftests/kvm: add test for KVM_GET_MSR_FEATURE_INDEX_LIST

From: Paolo Bonzini
Date: Wed Mar 17 2021 - 06:50:24 EST

On 17/03/21 08:45, Emanuele Giuseppe Esposito wrote:
+ struct kvm_msr_list features_list;
buffer.header.nmsrs = 1;
buffer.entry.index = msr_index;
+ features_list.nmsrs = 1;
kvm_fd = open(KVM_DEV_PATH, O_RDONLY);
if (kvm_fd < 0)
+ r = ioctl(kvm_fd, KVM_GET_MSR_FEATURE_INDEX_LIST, &features_list);
+ " rc: %i errno: %i", r, errno);

Careful: because this has nsmrs == 1, you are overwriting an u32 of the stack after struct kvm_msr_list. You need to use your own struct similar to what is done with "buffer.header" and "buffer.entry".

r = ioctl(kvm_fd, KVM_GET_MSRS, &buffer.header);
TEST_ASSERT(r == 1, "KVM_GET_MSRS IOCTL failed,\n"
" rc: %i errno: %i", r, errno);

More in general, this is not a test, but rather a library function used to read a single MSR.

If you would like to add a test for KVM_GET_MSR_FEATURE_INDEX_LIST that would be very welcome. That would be a new executable. Looking at the logic for the ioctl, the main purpose of the test should be:

- check that if features_list.nmsrs is too small it will set the nmsrs field and return -E2BIG.

- check that all MSRs returned by KVM_GET_MSR_FEATURE_INDEX_LIST can be accessed with KVM_GET_MSRS

So something like this:

set nmsrs to 0 and try the ioctl
check that it returns -E2BIG and has changed nmsrs
if nmsrs != 1 {
set nmsrs to 1 and try the ioctl again
check that it returns -E2BIG
malloc a buffer with room for struct kvm_msr_list and nmsrs indices
set nmsrs in the malloc-ed buffer and try the ioctl again
for each index
invoke kvm_get_feature_msr to read it

(The test should also be skipped if KVM does not expose the KVM_CAP_GET_MSR_FEATURES capability).