KVM: Allow kernel to select size of mmap() buffer

This allows us to store offsets in the kernel/user kvm_run area, and be
sure that userspace has them mapped.  As offsets can be outside the
kvm_run struct, userspace has no way of knowing how much to mmap.

Signed-off-by: Avi Kivity <avi@qumranet.com>

2 files changed, 11 insertions, 1 deletions

diff --git a/drivers/kvm/kvm_main.c b/drivers/kvm/kvm_main.c
index df85f5f65489..cba0b87c34e4 100644
--- a/drivers/kvm/kvm_main.c
+++ b/drivers/kvm/kvm_main.c
@@ -2436,7 +2436,7 @@ static long kvm_dev_ioctl(struct file *filp,
                           unsigned int ioctl, unsigned long arg)
 {
         void __user *argp = (void __user *)arg;
-        int r = -EINVAL;
+        long r = -EINVAL;
 
         switch (ioctl) {
         case KVM_GET_API_VERSION:
@@ -2478,6 +2478,12 @@ static long kvm_dev_ioctl(struct file *filp,
                  */
                 r = 0;
                 break;
+        case KVM_GET_VCPU_MMAP_SIZE:
+                r = -EINVAL;
+                if (arg)
+                        goto out;
+                r = PAGE_SIZE;
+                break;
         default:
                 ;
         }
diff --git a/include/linux/kvm.h b/include/linux/kvm.h
index c0d10cd8088e..dad90816cad8 100644
--- a/include/linux/kvm.h
+++ b/include/linux/kvm.h
@@ -253,6 +253,10 @@ struct kvm_signal_mask {
  * return is 1 (yes) or 0 (no, sorry).
  */
 #define KVM_CHECK_EXTENSION       _IO(KVMIO,   0x03)
+/*
+ * Get size for mmap(vcpu_fd)
+ */
+#define KVM_GET_VCPU_MMAP_SIZE    _IO(KVMIO,   0x04) /* in bytes */
 
 /*
  * ioctls for VM fds