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    SubjectRe: [PATCH 0/7] KVM: Kernel-based Virtual Machine
    On Sunday 22 October 2006 19:39, Anthony Liguori wrote:
    > 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.

    Right. It's a little tricky though regarding what happens when you
    write to the register mapping of a running guest, without stopping
    it first.

    > 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).

    what for? writing 1, then 0 to that file is two full syscalls.
    Calling kvm_run and returning from it is just one.

    You can also just send SIGSTOP/SIGCONT to the task to stop it.

    > 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.

    It's very handy to stay inside of a single process context for both
    the hypervisor and the guest, and to simply block in a kvm_run syscall
    for the time the guest executes.

    This syscall can then simply return the exit reason as its return
    value so you don't need another syscall to read it.

    > 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.

    I would not mix ioctls with a new file system. ioctl is fine on
    a character device, but with a new file system, you should be able
    to express everything as read/write, or one of the new syscalls.

    Arnd <><
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      Last update: 2006-10-22 19:57    [W:0.042 / U:209.468 seconds]
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