[PATCH v2] KVM: selftests: Make sure kvm_create_max_vcpus test won't hit RLIMIT_NOFILE

[Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>]

With the elevated 'KVM_CAP_MAX_VCPUS' value kvm_create_max_vcpus test
may hit RLIMIT_NOFILE limits:

 # ./kvm_create_max_vcpus
 KVM_CAP_MAX_VCPU_ID: 4096
 KVM_CAP_MAX_VCPUS: 1024
 Testing creating 1024 vCPUs, with IDs 0...1023.
 /dev/kvm not available (errno: 24), skipping test

Adjust RLIMIT_NOFILE limits to make sure KVM_CAP_MAX_VCPUS fds can be
opened. Note, raising hard limit ('rlim_max') requires CAP_SYS_RESOURCE
capability which is generally not needed to run kvm selftests (but without
raising the limit the test is doomed to fail anyway).

Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
---
Changes since v1:
- Drop 'NOFD' define replacing it with 'int nr_fds_wanted' [Sean]
- Drop 'errno' printout as TEST_ASSERT() already does that.
---
 .../selftests/kvm/kvm_create_max_vcpus.c      | 22 +++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/tools/testing/selftests/kvm/kvm_create_max_vcpus.c b/tools/testing/selftests/kvm/kvm_create_max_vcpus.c
index f968dfd4ee88..ca957fe3f903 100644
--- a/tools/testing/selftests/kvm/kvm_create_max_vcpus.c
+++ b/tools/testing/selftests/kvm/kvm_create_max_vcpus.c
@@ -12,6 +12,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/resource.h>
 
 #include "test_util.h"
 
@@ -40,10 +41,31 @@ int main(int argc, char *argv[])
 {
 	int kvm_max_vcpu_id = kvm_check_cap(KVM_CAP_MAX_VCPU_ID);
 	int kvm_max_vcpus = kvm_check_cap(KVM_CAP_MAX_VCPUS);
+	/*
+	 * Number of file descriptors reqired, KVM_CAP_MAX_VCPUS for vCPU fds +
+	 * an arbitrary number for everything else.
+	 */
+	int nr_fds_wanted = kvm_max_vcpus + 100;
+	struct rlimit rl;
 
 	pr_info("KVM_CAP_MAX_VCPU_ID: %d\n", kvm_max_vcpu_id);
 	pr_info("KVM_CAP_MAX_VCPUS: %d\n", kvm_max_vcpus);
 
+	/*
+	 * Check that we're allowed to open nr_fds_wanted file descriptors and
+	 * try raising the limits if needed.
+	 */
+	TEST_ASSERT(!getrlimit(RLIMIT_NOFILE, &rl), "getrlimit() failed!");
+
+	if (rl.rlim_cur < nr_fds_wanted) {
+		rl.rlim_cur = nr_fds_wanted;
+
+		if (rl.rlim_max <  nr_fds_wanted)
+			rl.rlim_max = nr_fds_wanted;
+
+		TEST_ASSERT(!setrlimit(RLIMIT_NOFILE, &rl), "setrlimit() failed!");
+	}
+
 	/*
 	 * Upstream KVM prior to 4.8 does not support KVM_CAP_MAX_VCPU_ID.
 	 * Userspace is supposed to use KVM_CAP_MAX_VCPUS as the maximum ID
-- 
2.33.1