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    SubjectRe: [PATCH 1/7] KVM: userspace interface
    Avi Kivity wrote:
    > Anthony Liguori wrote:
    >>>
    >>> writel(dst_x_reg, x);
    >>> writel(dst_y_reg, y)
    >>> writel(width_reg, w);
    >>> writel(height_reg, h);
    >>> writel(blt_cmd_reg, fill);
    >>>
    >>> then kvm would cache the first four in a mmap()able memory area and
    >>> only exit to userspace on the fifth. Userspace would then read the
    >>> cached registers from memory and emulate the command.
    >>
    >> Letting QEMU do a certain amount of emulation after every transition
    >> would the problem in a more elegant and generic way.
    >>
    >
    > But what amount? A basic block, or several?
    >
    > Emulation has its costs. You need to marshal the registers to and
    > fro. You need to reset qemu's cached translations. You need to throw
    > away shadow page tables and qemu's softmmu. You increase the time
    > spent in single threaded code.

    Admittedly still a research topic. If you're interested in what we're
    doing in Xen, check out:

    http://xenbits.xensource.com/ext/xen-unstable-hvm.hg (sorry, xenbits is
    down right now but hopefully it will be fixed quickly).

    Regards,

    Anthony Liguori

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