Re: [PATCH 0/7] KVM: Kernel-based Virtual Machine

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Avi Kivity wrote:

Arnd Bergmann wrote:
This looks _a_lot_ like what we're doing for the SPUs in the cell processor, 
except that we're using different calls into the kernel. Have you looked
into what we have implemented there? The code is in
arch/powerpc/platforms/cell/spufs. I think it would be a good abstraction to use for you as well, maybe we could even move to a common infrastructure, as I have heard from a few other projects that want to do similar things. 

The main differences to your interface are:

- A file system is used instead of a character device
- Directories, not open file descriptors represent contexts
- Two new syscalls were introduced (spu_create/spu_run)
- instead of ioctls, files represent different bits of information,
  you can read/write, poll or mmap them.

Your example above could translate to something like:

   int kvm_fd = kvm_create("/kvm/my_vcpu")
   int mem_fd = openat(kvm_fd, "mem", O_RDWR);
   void *mem = mmap(mem_fd, ...); // main memory
   void *fbmem = mmap(mem_fd, ...); // frame buffer memory
   int regs_fd = openat(kvm_fd, "regs", O_RDWR);
   int irq_fd = openat(kvm_fd, "regs", O_WRONLY);

   if (debugger) {
     int fd = openat(fvm_fd, "debug", O_WRONLY);
     write(fd, "1", 1);
     close(fd);
   }
   while (1) {
      int exit_reason = kvm_run(kvm_fd, &kvm_descriptor);
      switch (exit reason) {
          handle mmio, I/O etc. might call
write(irq_fd, &interrupt_packet, sizeof (interrupt_packet)); 
             pread(regs_fd, &rax, sizeof rax, KVM_REG_RAX);
   }


[cc'ing some others to solicit their opinion]
I like this. Since we plan to support multiple vcpus per vm, the fs structure might look like:
I like the idea of a filesystem. In particular, if you exposed the CPU state as a mmap()'able file, you could read/write from userspace without any syscall overhead.
There are some clever ways that you could get around need that many syscalls. For instance, you could have a "paused" file that you could write a "1" into in order to run the guest (assuming that the memory/CPU state is setup properly).
You could then have an "event" file that you could select() for read on. When "event" became readable, you could read the exit reason, do whatever is needed, and then write a "1" into "paused" again.
Perhaps an ioctl is better for pausing/unpausing but I do think it's necessary to select() on something to wait for the next exit reason to occur. 

Regards,

Anthony Liguori


/kvm/my_vm
   |
   +----memory          # mkdir to create memory slot.
   |     |              #    how to set size and offset?
   |     |
   |     +---0          # guest physical memory slot
   |         |
   |         +-- dirty_bitmap  # read to get and atomically reset
   |                           # the changed pages log
   |
   |
   +----cpu             # mkdir/rmdir to create/remove vcpu
         |
         +----0
         |     |
         |     +--- irq     # write to inject an irq
         |     |
         |     +--- regs    # read/write to get/set registers
         |     |
         |     +--- debugger   # write to set breakpoints/singlestep mode
         |
         +----1
               [...]
It's certainly a lot more code though, and requires new syscalls. Since this is a little esoteric does it warrant new syscalls? 



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