kvm: embed vcpu id to dentry of vcpu anon inode

All d-entries for vcpu have the same, "anon_inode:kvm-vcpu". That means
it is impossible to know the mapping between fds for vcpu and vcpu
from userland.

    # LC_ALL=C ls -l /proc/617/fd | grep vcpu
    lrwx------. 1 qemu qemu 64 Jan  7 16:50 18 -> anon_inode:kvm-vcpu
    lrwx------. 1 qemu qemu 64 Jan  7 16:50 19 -> anon_inode:kvm-vcpu

It is also impossible to know the mapping between vma for kvm_run
structure and vcpu from userland.

    # LC_ALL=C grep vcpu /proc/617/maps
    7f9d842d0000-7f9d842d3000 rw-s 00000000 00:0d 20393                      anon_inode:kvm-vcpu
    7f9d842d3000-7f9d842d6000 rw-s 00000000 00:0d 20393                      anon_inode:kvm-vcpu

This change adds vcpu id to d-entries for vcpu. With this change
you can get the following output:

    # LC_ALL=C ls -l /proc/617/fd | grep vcpu
    lrwx------. 1 qemu qemu 64 Jan  7 16:50 18 -> anon_inode:kvm-vcpu:0
    lrwx------. 1 qemu qemu 64 Jan  7 16:50 19 -> anon_inode:kvm-vcpu:1

    # LC_ALL=C grep vcpu /proc/617/maps
    7f9d842d0000-7f9d842d3000 rw-s 00000000 00:0d 20393                      anon_inode:kvm-vcpu:0
    7f9d842d3000-7f9d842d6000 rw-s 00000000 00:0d 20393                      anon_inode:kvm-vcpu:1

With the mappings known from the output, a tool like strace can report more details
of qemu-kvm process activities. Here is the strace output of my local prototype:

    # ./strace -KK -f -p 617 2>&1 | grep 'KVM_RUN\| K'
    ...
    [pid   664] ioctl(18, KVM_RUN, 0)       = 0 (KVM_EXIT_MMIO)
     K ready_for_interrupt_injection=1, if_flag=0, flags=0, cr8=0000000000000000, apic_base=0x000000fee00d00
     K phys_addr=0, len=1634035803, [33, 0, 0, 0, 0, 0, 0, 0], is_write=112
    [pid   664] ioctl(18, KVM_RUN, 0)       = 0 (KVM_EXIT_MMIO)
     K ready_for_interrupt_injection=1, if_flag=1, flags=0, cr8=0000000000000000, apic_base=0x000000fee00d00
     K phys_addr=0, len=1634035803, [33, 0, 0, 0, 0, 0, 0, 0], is_write=112
    ...
Signed-off-by: Masatake YAMATO <yamato@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

virt/kvm/kvm_main.c

@@ -2415,7 +2415,10 @@ static struct file_operations kvm_vcpu_fops = {
  */
 static int create_vcpu_fd(struct kvm_vcpu *vcpu)
 {
-	return anon_inode_getfd("kvm-vcpu", &kvm_vcpu_fops, vcpu, O_RDWR | O_CLOEXEC);
+	char name[8 + 1 + ITOA_MAX_LEN + 1];
+
+	snprintf(name, sizeof(name), "kvm-vcpu:%d", vcpu->vcpu_id);
+	return anon_inode_getfd(name, &kvm_vcpu_fops, vcpu, O_RDWR | O_CLOEXEC);
 }
 
 static int kvm_create_vcpu_debugfs(struct kvm_vcpu *vcpu)