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    SubjectRe: [PATCH 2/5] KVM: SVM: Optimize nested svm msrpm merging
    On 02/26/2010 02:25 PM, Joerg Roedel wrote:
    > On Fri, Feb 26, 2010 at 12:28:24PM +0200, Avi Kivity wrote:
    >
    >>> +static void add_msr_offset(u32 offset)
    >>> +{
    >>> + u32 old;
    >>> + int i;
    >>> +
    >>> +again:
    >>> + for (i = 0; i< MSRPM_OFFSETS; ++i) {
    >>> + old = msrpm_offsets[i];
    >>> +
    >>> + if (old == offset)
    >>> + return;
    >>> +
    >>> + if (old != MSR_INVALID)
    >>> + continue;
    >>> +
    >>> + if (cmpxchg(&msrpm_offsets[i], old, offset) != old)
    >>> + goto again;
    >>> +
    >>> + return;
    >>> + }
    >>> +
    >>> + /*
    >>> + * If this BUG triggers the msrpm_offsets table has an overflow. Just
    >>> + * increase MSRPM_OFFSETS in this case.
    >>> + */
    >>> + BUG();
    >>> +}
    >>>
    >> Why all this atomic cleverness? The possible offsets are all
    >> determined statically. Even if you do them dynamically (makes sense
    >> when considering pmu passthrough), it's per-vcpu and therefore
    >> single threaded (just move msrpm_offsets into vcpu context).
    >>
    > The msr_offset table is the same for all guests. It doesn't make sense
    > to keep it per vcpu because it will currently look the same for all
    > vcpus. For standard guests this array contains 3 entrys. It is marked
    > with __read_mostly for the same reason.
    >

    In that case, you can calculate it during module initialization.

    --
    Do not meddle in the internals of kernels, for they are subtle and quick to panic.



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