KVM: provide synchronous registers in kvm_run

On some cpus the overhead for virtualization instructions is in the same
range as a system call. Having to call multiple ioctls to get set registers
will make certain userspace handled exits more expensive than necessary.
Lets provide a section in kvm_run that works as a shared save area
for guest registers.
We also provide two 64bit flags fields (architecture specific), that will
specify
1. which parts of these fields are valid.
2. which registers were modified by userspace

Each bit for these flag fields will define a group of registers (like
general purpose) or a single register.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

Documentation/virtual/kvm/api.txt

@@ -1771,6 +1771,29 @@ developer registration required to access it).
 		/* Fix the size of the union. */
 		char padding[256];
 	};
+
+	/*
+	 * shared registers between kvm and userspace.
+	 * kvm_valid_regs specifies the register classes set by the host
+	 * kvm_dirty_regs specified the register classes dirtied by userspace
+	 * struct kvm_sync_regs is architecture specific, as well as the
+	 * bits for kvm_valid_regs and kvm_dirty_regs
+	 */
+	__u64 kvm_valid_regs;
+	__u64 kvm_dirty_regs;
+	union {
+		struct kvm_sync_regs regs;
+		char padding[1024];
+	} s;
+
+If KVM_CAP_SYNC_REGS is defined, these fields allow userspace to access
+certain guest registers without having to call SET/GET_*REGS. Thus we can
+avoid some system call overhead if userspace has to handle the exit.
+Userspace can query the validity of the structure by checking
+kvm_valid_regs for specific bits. These bits are architecture specific
+and usually define the validity of a groups of registers. (e.g. one bit
+ for general purpose registers)
+
 };
 
 6. Capabilities that can be enabled