[Qemu-devel] [RFC] Programmable guest-to-QEMU hypercalls

[Lluís Vilanova]

Hi! I have in my trace instrumentation queue a series that adds a very simple
but efficient way to trigger code in QEMU from guest code using guest-agnostic
code.

Blue Swirl showed some interest long ago in using it in the test suite (e.g.,
instruct QEMU to check the vCPU state after a series of instructions). But I
don't know if there still is interest, or if anybody else finds this useful
(otherwise I'll keep it in my instrumentation branch).

Guest-side interface:

  #include <qemu-hypercall.h>
  int main()
  {
      // initialize communication device
      qemu_hypercall_init("/tmp/hypercall");

      // memory region to share data between guest and QEMU
      // (QEMU does not trap reads/writes here, so it can be used as a
      // bandwidth-efficient communication channel)
      void *data = qemu_hypercall_data();
      ((char*)data)[0] = 0x1;

      // trigger hypercall callback
      qemu_hypercall(0xcafe);           // in-line data
  }

A dynamic library is loaded when starting QEMU, which gets called as a response
to 'qemu_hypercall()':

  // libmyhypercall.so
  qemu_hypercall(uint64_t cmd, char *data)
  {
      assert(cmd == 0xcafe)
      assert(((char*)data)[0] == 0x1);
  }

To start QEMU:

  qemu-x86_64 -hypercall libmyhypercall.so -hypercall-device=/tmp/backdoor  
/test/program
  qemu-system-x86_64 -device hypercall

I have a prototype for a guest user library and a guest Linux module to use this
in both user and system mode.


Cheers,
  Lluis