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    SubjectRe: [ PATCH 2.6.16-rc3-xen 3/3] sysfs: export Xen hypervisor attributes to sysfs
    Arjan van de Ven wrote:
    > On Wed, 2006-02-22 at 08:43 -0500, Mike D. Day wrote:
    >
    >> Heiko Carstens wrote:
    >>
    >>> On Wed, Feb 22, 2006 at 08:06:12AM -0500, Mike D. Day wrote:
    >>>
    >>> If it's not needed, why include it at all?
    >>>
    >> Sorry for not being clear. It *is* needed for control tools and agents
    >> running in the privileged domain.
    >>
    >
    > but again those tools and agents *already* have a way of talking to the
    > hypervisor themselves. Why can't they just first ask this info? Why does
    > that need to be in the kernel, in unswappable memory?
    >
    Hypercalls have to be done in ring 0 for security reasons) There has to
    be some kernel interface for making hypercalls.

    The current interface is a ioctl() on a /proc file (which is awful).
    The ioctl just pretty much passes 5 word arguments to the hypervisor.
    It was suggested previously here that a hypercall pass-through interface
    isn't the right approach. One suggestion that came up was a syscall
    interface.

    Also, there are some kernel-level drivers, like the memory ballooning
    driver, that only exist in the kernel. Controlling the balloon driver
    requires some sort of interface. That was the original point of this
    effort (since it's currently exposed as a /proc file). I think it's
    quite clear that the balloon driver should expose itself through sysfs
    but I'm not personally convinced that this information (hypervisor
    version information) ought to be exposed in sysfs.

    Regards,

    Anthony Liguori
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