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

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Message-ID: <453B2DDB.3010303@qumranet.com>
Date:	Sun, 22 Oct 2006 10:37:47 +0200
From:	Avi Kivity <avi@...ranet.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Muli Ben-Yehuda <muli@...ibm.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Anthony Liguori <aliguori@...ibm.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH 0/7] KVM: Kernel-based Virtual Machine

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:

/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?

-- 
error compiling committee.c: too many arguments to function

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