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    SubjectRe: [PATCH] Add Documentation/kvm/msr.txt
    On 05/27/2010 06:02 AM, Glauber Costa wrote:
    > On Thu, May 27, 2010 at 11:15:43AM +0300, Avi Kivity wrote:
    >
    >> On 05/26/2010 09:04 PM, Glauber Costa wrote:
    >>
    >>> This patch adds a file that documents the usage of KVM-specific
    >>> MSRs.
    >>>
    >>>
    >> Looks good. A few comments:
    >>
    >>
    >>> +
    >>> +Custom MSR list
    >>> +--------
    >>> +
    >>> +The current supported Custom MSR list is:
    >>> +
    >>> +MSR_KVM_WALL_CLOCK: 0x11
    >>> +
    >>> + data: physical address of a memory area.
    >>>
    >> Which must be in guest RAM (i.e., don't point it somewhere random
    >> and expect the hypervisor to allocate it for you).
    >>
    >> Must be aligned to 4 bytes (we don't enforce it though).
    >>
    > I don't see the reason for it.
    >
    > If this is a requirement, our own implementation
    > is failing to meet it.
    >

    It's so the atomic write actually is atomic. Stating a 4 -byte
    alignment requirement prevents the wall clock from crossing a page boundary.

    Zach



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