For reading and writing guest user space buffers, currently the kernel
sends the guest virtual address of the buffer to the pipe device. This
virtual address has to be first converted to a guest physical address.
Doing this translation on the QEMU side is inefficient and requires
additional handling when KVM is enabled, whose implementation would
either incur intrusive changes to QEMU's KVM support code or suffer
from poor performance, see commit 08c7228c50f8 ("x86-kvm: only sync
SREGS when doing address translation") of $AOSP/external/qemu for
details, and thus should be avoided if possible.

There is a TODO comment in hw/misc/android_pipe.c in the new Android
emulator source tree ($AOSP/external/qemu-android) which requests that
the translation be done on the kernel side and that physical addresses
be passed to the device instead of virtual ones. Once the QEMU-side
implementation is done, the kernel will need to support both the new
paddr-based pipe device and the old vaddr-based one (which will
continue to be used by the classic emulator). This patch achieves that
by leveraging the device version register available in the new device.

See https://android-review.googlesource.com/128280 for the QEMU-side
patch.

In addition, use the mmap semaphore (in read mode) to safeguard the
call to get_user_pages().
Signed-off-by: Yu Ning <yu.ning@intel.com>
Signed-off-by: Jin Qian <jinqian@android.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>